Roger Federer turned 40 on Sunday amid uncertainty over whether he will grace the stage of a grand slam again.

Both he and Serena Williams, who reaches the same birthday landmark in September, have kept their future plans under wraps.

However, it would come as no surprise now if one, or both, were to retire by the end of the year.

Injuries are taking their toll, and even the greatest champions cannot go on forever.

Stats Perform looked at both Federer and Williams, considering what they may still want to achieve, and their prospects of attaining those remaining goals.

Federer's final fling?

Ahead of his 30th birthday, Federer was asked what it felt like to hit such a milestone.

"Birthdays happen. They're part of life," Federer said. "I'm happy I'm getting older. I'd rather be 30 than 20, to be honest. To me it's a nice time."

A decade on, Federer has good reason to be satisfied with life as he chalks up another decade. Family life is good, he'll never need to borrow a dollar, and he has advanced from 16 grand slams to 20.

But the knees would sooner be 30 than 40, and Federer, remarkable sportsman though he is, looks to have entered the lap of honour stage of his career – if he can even complete such a lap.

Two knee operations in 2020 were followed by a setback that ruled him out of the Olympics and will also keep him sidelined for the Toronto and Cincinnati tournaments before the US Open.

Will Federer make it to Flushing Meadows, where he won five successive titles at the height of his career? There has to be doubt over that, and should he indeed be an absentee in New York, what is there left to target? The Laver Cup, perhaps, a tournament in which he is financially invested and which is due to be played in Boston in late September.

Would he play on in 2022? Could he tolerate more long road trips without his family, living in a tennis bubble?

Target: Federer has never settled for second best, and it may have dawned on him at Wimbledon that in all probability he no longer can win a grand slam. Losing a 6-0 set to Hubert Hurkacz on the way to a quarter-final exit would have hurt. The hunger does not go away after 20 grand slams, but Federer's battle-weary body is sending him messages. He will want to go out on his own terms, which means getting fully fit.

Prospects: Assuming the knee issue is not a major problem, and more of a niggle, then Federer could still play the US Open, Laver Cup, Indian Wells and Paris Masters this year. If the mind is willing but the body does not comply, however, then it would not be a shock to see him call time before the Australian Open comes around in January.

Serena still one short of Court

From precocious teenager to queen of the tour, Williams' tennis journey has been a 25-year odyssey and there is nobody more driven to succeed than the great American.

It is an intense frustration that she remains rooted on 23 grand slams, one short of Margaret Court's record haul, and the four grand slam final losses she has suffered while on that mark have been cruel blows.

As her 40th birthday approaches on September 26, prospects of matching Court are fading. The leg injury that cruelly forced her out of Wimbledon in the first round was a harrowing turn of events, given she looked primed to be a big title challenger in London.

She is becoming less of a factor when looking at title favourites, but Williams is still capable of beating top players, still a threat wherever she shows up. It comes down to whether the body lets her chase her goals, and whether the pain of so many near-misses in recent years persuades this great champion the exertion is no longer worth prolonging.

Target: The 24th slam has been the must-have for Williams. Tour titles feel like an irrelevance, and Williams has won just one of those since January 2017, her calendar built around peaking for the majors since returning from giving birth to daughter Olympia.

Prospects: Beating Aryna Sabalenka and Simona Halep at the Australian Open demonstrated Williams still has the game for the big stage, and a semi-final defeat to Naomi Osaka, to whom she has now lost in three of four encounters, should not particularly detract from that. Wimbledon felt like a golden opportunity, with a host of major rivals absent and others struggling for form. There is no doubt she felt that way. Getting to 24 – and beyond – has shifted from feeling like an inevitability to being an odds-against chance now.

Novak Djokovic led the ATP Tour's birthday message to Roger Federer, as the Swiss star celebrated his 40th on Sunday.

Federer, ranked ninth in the world, made his professional debut in 1998. He has gone on to win 20 grand slam titles, a feat matched by only Rafael Nadal and Djokovic, who tied his rivals with his triumph at Wimbledon in July.

His first grand slam title arrived at Wimbledon in 2003, when Federer was aged just 21. In 2004, 2006 and 2007, he won three out of the four majors on offer.

Federer has six Australian Open titles, eight Wimbledon triumphs, five US Open wins and one French Open success to his name, while he has also won six ATP Tour Finals.

Djokovic and Federer have met 50 times on the ATP circuit, with the Serbian shading their head-to-head record with 27 wins – their most recent encounter coming in the semi-finals of the 2020 Australian Open.

Yet it was he who led the tributes to a true great.

"Hey Roger, happy 40th birthday – 40th, wow, what a milestone! You still keep on inspiring and thriving on the court, off the court, inspiring all of us," Djokovic said in a video posted to the ATP Tour's official Twitter account.

"It's been a huge honour to share the tennis court and the tennis circuit with you in the last 15 years.

"Hopefully you can still keep on playing. The sport needs you, of course, and thanks for everything you have done and thanks for showing us that even at that age we can play at a very high level.

"All the best, I wish you health and love and happiness with your close ones and thanks again for everything."

Kei Nishikori, who has beaten Federer three times and suffered eight defeats to the Swiss, was next up.

"Roger, happy 40th birthday!" Nishikori said.

"You are still my idol, I always looked up to you and I hope we can play a couple more times and I hope you can still keep going and win grand slams."

World number 15 Felix Auger Aliassime shares his birthday with Federer, though the Canadian is some 19 years junior.

"Hey Roger, the big 40, while I'm just turning 21," he said. "It's amazing for the sport that I'm playing and you're still playing at the same time.

"I hope I'll still be playing when I'm 40 as well. Thank you for everything you've been doing for tennis, it's so good to still have you around, I hope you have a great birthday."

American Mackenzie McDonald toppled former champion Kei Nishikori in three sets to claim a spot in the Citi Open final on Sunday against fifth seed Jannik Sinner.

McDonald triumphed over unseeded Japanese Nishikori 6-4 3-6 7-5 in two hours and 45 minutes in Washington DC.

The 26-year-old American's win qualifies him for his maiden ATP Tour final, sending down 4-1 aces while getting a slight edge with his return.

Nishikori, who triumphed in DC in 2015, fought back when trailing a set and a break in the second with four straight games to send it to a third.

The 31-year-old Japanese appeared to have the momentum but McDonald responded to outlast him.

McDonald will face 19-year-old Sinner who overcame another emerging talent, American 20-year-old Jenson Brooksby, 7-6 (7-2) 6-1.

Sinner becomes the first Italian to reach the final in DC and also progresses to his first ATP 500 decider.

The Italian continued his fine run in DC, having not yet dropped a set all tournament, marking a return to form after being on a four-match losing run.

Sinner saved three set points at 5-6 in the first to fight back to take the lead in a tiebreaker before a dominant second.

Kei Nishikori is into his first ATP Tour semi-final in more than two years after defeating Lloyd Harris in straight sets at the Citi Open on Friday. 

By far the most experienced player remaining in the field after Harris' upset of Rafael Nadal on Thursday, Nishikori prevailed 6-3 7-5 to avenge a loss earlier this year in their only prior meeting. 

Nishikori had lost his last six ATP quarter-final matches dating to a trip to the semis in Barcelona in April 2019. 

The 31-year-old next faces Mackenzie McDonald, who downed countryman Denis Kudla 6-3 6-2 to reach his second career ATP semi-final. He has never played in a final. 

In the other semi-final, fifth seed Jannik Sinner will meet wild card Jenson Brooksby. 

Sinner had little trouble with American veteran Steve Johnson in a 6-4 6-2 win as the Italian won 93 per cent of his first-serve points and saved all three break points he faced. 

Brooksby maintained the strong return game that as carried him all week in a 6-1 6-2 upset of 11th seed John Millman. 

The Australian did not manage an ace against the 20-year-old American, who won 54 per cent of the points on Millman's serve and converted five of seven break points. 

At 130 in the world, Brooksby is the lowest-seeded Citi Open semi-finalist since John Isner reached the 2007 final while ranked 416th.

This time, Rafael Nadal could not summon the third-set magic he needed.

Coming off an epic three-hour duel Wednesday in his first match since the French Open final, Nadal fell to Lloyd Harris 6-4 1-6 6-4 on Thursday at the Citi Open. 

After a short-lived maiden appearance at the Washington, D.C. tournament, the Spaniard will look ahead to Toronto and Cincinnati as he continues his preparations for the U.S. Open. 

Harris, meanwhile, heads to a quarter-final matchup against veteran Kei Nishikori after recording the biggest win of his career. 

Facing Nadal for the first time, the 24-year-old South African slammed 16 aces Thursday and saved four of six break points. 

The 20-time grand slam winner, who battled a foot injury to outlast Jack Sock in his first match, did not have enough to answer the world number 50 on Thursday.

In addition to the top-seeded Nadal's defeat, second seed Felix Auger-Aliassime also went down Thursday, falling 6-3 6-4 to American wild card Jenson Brooksby. 

With their exits, the tournament is now without its top four seeds. 

That leaves fifth seed Jannik Sinner as the top player remaining after he defeated Sebastian Korda 7-6 (7-3) 7-6 (7-3). 

Nishikori advanced by downing seventh seed Cameron Norrie 3-6 6-3 6-3, while John Millman knocked out eighth seed Reilly Opelka 6-3 7-6 (7-4). 

Among unseeded players, Denis Kudla defeated the red-hot Brandon Nakashima 3-6 7-6 (7-3) 6-4, while Steve Johnston eased past Ricardas Berankis 6-2 6-1 and Mackenzie McDonald beat Illya Ivashka 6-4 6-4. 

In addition to Harris-Nishikori, Kudla will face McDonald in the other quarter-final in the top half of the draw, while Sinner meets Johnson and Millman takes on Brooksby in the bottom half. 

World number three Rafael Nadal outlasted Jack Sock in a three-hour epic to progress through to the third round at the Citi Open in Washington D.C. on Wednesday.

The 20-time Grand Slam winner needed a third-set tiebreaker to beat American wildcard Sock 6-2 4-6 7-6 (7-1).

Sock, who is currently ranked 192nd in the world, stormed back into the match with an early break in the second set after the Spaniard cruised in the first.

In a three-hour-and-a-four-minute match which saw only five breaks, Nadal got the edge winning more on his second serve, while he produced some vintage winners too.

Nadal goes through to the third round where he will face 14th seed Lloyd Harris, who progresses after a walkover with Tennys Sandgren retiring mid-match.

Third seed and world number 18 Alex De Minaur was bundled out by world number 81 Steve Johnson 6-7 (5-7) 6-4 6-2.

Fourth seed Grigor Dimitrov also was dumped out with a 6-2 7-6 (7-4) defeat to Belarussian world number 65 Ilya Ivashka.

Sixth seed Dan Evans went out in straight sets to 20-year-old American Brandon Nakashima 7-6 (7-1) 6-0 while 10th seed Taylor Fritz also bowed out to Denis Kudla 6-4 6-2.

Former winner Kei Nishikori knocked off ninth seed Alexander Bublik 6-2 7-5, while 13th seed Benoit Paire lost in three sets to Mackenzie McDonald.

Fifth seed Jannik Sinner got through in straight sets against Emil Ruusuvuori, along with eighth seed Reilly Opelka and 11th seed John Millman.

Second seed Felix Auger-Aliassime survived after an early scare against Andreas Seppi to progress to the third round at the Citi Open in Washington D.C. on Tuesday.

The Canadian world number 15 won 2-6 6-2 6-2 in just under two hours over the 37-year-old Italian.

Seppi had beaten Auger-Aliassime in the opening round at the French Open in June.

Auger-Aliassime this time hit 25 winners, including nine aces, overcoming a slow start where he was broken twice inside the opening six games to win in three sets.

Youngster Jenson Brooksby continued his bright start in Washington D.C. with a 7-6 (7-4) 7-5 win over fellow American Francis Tiafoe.

Australian Nick Kyrgios was bundled out by Mackenzie McDonald 6-4 6-4, while 12th seed Sebastian Korda eased past Vasek Pospisil 7-5 6-4 in the second round.

Seventh seed Cameron Norrie also triumphed in straight sets over Marcos Giron 6-4 6-3, while emerging star Brandon Nakashima celebrated his 20th birthday with a 6-3 6-3 victory over Alexei Popyrin.

Denis Kudla beat Feliciano Lopez to set up a second round date with 10th seed Taylor Fritz, while Tennys Sandgren will meet 14th seed Lloyd Harris after beating James Duckworth in three sets.

Finland's Emil Ruusuvuori beat India's Prajnesh Gunneswaran 2-6 6-1 6-1 and will take on fifth seed Jannik Sinner in the second round.

Young American Jenson Brooksby claimed some revenge for last month's Hall of Fame Open final defeat to Kevin Anderson by beating the South African 7-6 (7-4) 6-3 in the Citi Open first round on Monday.

Brooksby, who received a wildcard for the draw, triumphed in one hour and 49 mnutes against the former world number five.

The 20-year-old American saved a set point at 5-6 in the first set, before winning in a tiebreaker off the back of winning six consecutive points having trailed 1-4.

Former City Open finalist Anderson, who sent down 10-1 aces, had defeated Brooksby to win the Hall of Fame Open last month.

World number 130 Brooksby, who had started the season outside the top 300, claimed the crucial break in the eighth game of the second set.

Brooksby advances to take on American Francis Tiafoe, fresh from competing at Tokyo 2020, as numerous big names enter the draw for the second round.

Kei Nishikori, who won in Washington in 2015, got past American Sam Querrey 6-4 6-3 to set up a second round meeting with Kazakhstani ninth seed Alexander Bublik.

American Jack Sock will take on top seed Rafael Nadal in the second round after he got past Yoshihito Nishioka in a walkover 6-7 (3-7) 4-0.

Former top 20 player Andreas Seppi triumphed in three sets against Yasutaka Uchiyama while Canadian tour veteran Vasek Pospisil got past Emilio Gomez in three.

Ilya Ivashka, Marcos Giron, Ricardas Berankas and Daniel Galan were the other first round winners on Monday.

John Isner has claimed his sixth Atlanta Open title after a straight sets win over American teenager Brandon Nakashima in Sunday's final.

Isner, who had won five of the past seven Atlanta Opens held, triumphed 7-6 (10-8) 7-5 in one hour and 56 minutes.

The victory is world number 35 Isner's 16th ATP Tour career title. It is also 36-year-old Isner's first title since 2018.

Isner sent down 21 aces on route to the win, where his serve was not broken once with Nakashima unable to convert any of his three break points.

The 208cm American broke for the first time in the crucial game, deciding the contest with Nakashima dishing up a double fault on match point.

Nakashima had beaten Isner in last week's Los Cabos Open semi-finals but the veteran has made the Atlanta court his own.

The 19-year-old had one set point in the first set tiebreaker which he failed to convert before Isner capitalized with his third, winning 10-8.

Isner's dominance was on his serve, winning 81 per cent of first serve points, which he landed 58 of 77, with break points rare until the final three games.

The big American had three break points at 5-4 in the second set but Nakashima saved them all, before the teenager let slip two break points in the next game.

Isner converted a single break point in the 12th game of the second set to salute.

Alexander Zverev followed up his stunning win over Novak Djokovic by storming to Olympic gold in the men's singles final.

A thumping 6-3 6-1 victory against Russian Olympic Committee's Karen Khachanov was the perfect way for world number five Zverev to land the biggest title of his career.

He won 10 of the last 11 games of his semi-final against world number one Djokovic and carried that form into Sunday's title match, brushing aside the threat across the net.

It makes him Germany's first men's singles champion at the Olympics. Steffi Graf won the women's title as part of a calendar Golden Slam in 1988, and four years later Boris Becker and Michael Stich teamed up to take doubles gold.

For the man who won the ATP Finals in 2018 and reached last year's US Open title match, this marked a step forward in a career many expect will eventually feature grand slam titles.

He secured a swift break in the first set against Khachanov, who was outstanding in the semi-finals against Pablo Carreno Busta but could not impose his game in the gold-medal match.

Another break followed and the set was gone in 43 minutes.

The second set rushed by in a mere 36 minutes, with Zverev against striking early and never looking back.

He created two break points in the second game of that set, and although Khachanov saved them, a third soon followed and the Russian netted on the forehand.

The momentum was all one way and there was never a moment when Khachanov, ranked 25th by the ATP, looked as though he might pose some danger. 

Zverev gave himself a first match point when he punched away a backhand volley, and the glory belonged to the German when Khachanov slapped a forehand into the net on the next point.

BENCIC DENIED GOLDEN DOUBLE

Belinda Bencic was the surprise champion in the women's singles and the Swiss had a chance to land a second gold medal on Sunday when she and Swiss partner Viktorija Golubic lined up in the doubles final.

It was not to be for Bencic and Golubic, however, as they were soundly beaten by the prolific Czech pair of Barbora Krejcikova and Katerina Siniakova.

A 7-5 6-1 win for the reigning French Open champions meant Bencic and Golubic were left with the silver medal.

But there was little in the way of regret for the Swiss pair, with Bencic showing off both her gold and silver medals after the match, saying: "We are overjoyed. We will fly back on Monday with other medallists. It's a great feeling to know people are waiting and they acknowledge the medals we won for our country.

"The week here was incredibly beautiful. We experienced so many emotions, and it's great to go back and share all of these emotions with everyone at home.

"It's not just about the medals or the titles, it's about the memories you create that will last forever. To share this with Viki is unbelievable.

"The whole week I never felt like I was in a normal tournament or playing alone. She was alongside me the entire way. I always tell her we won this gold medal together as well. When we will be 80 years old and have a coffee, we will talk about this moment and I cannot wait for that."

One-third of John Isner's 15 career ATP titles have come at the Atlanta Open and the American will play for another Sunday against an up-and-coming countryman. 

Isner defeated Tayor Fritz 7-6 (7-4) 5-7 6-3 on Saturday to reach his first ATP Tour final this year after losing in the semis three times. 

One of those semi-final setbacks came last week in Los Cabos to Brandon Nakashima, who will be Isner's opponent Sunday. 

The 19-year-old Californian, ranked a career-best 115th, rallied to beat Finland's Emil Ruusuvuori 3-6 6-4 6-3 in the other semi. 

Nakashima defeated top seed Milos Raonic earlier this week and is riding high as he looks for his first ATP title. 

He is the first US teenager to reach the final in consecutive weeks since 18-year-old Andy Roddick won his first two ATP tournaments at Atlanta and Houston in the spring of 2001. 

Roddick's maiden title came in an earlier version of the Atlanta event. Since it was revived in 2010, Isner has won five of the 10 titles, with the first coming in 2013 about a week before Nakashima's 12th birthday.

"I didn’t know much about [Nakashima] prior to last week," Isner told the ATP's website. "But he kicked my ass last week, so we’ll see what I can do tomorrow.

"He’s 19 years old, that’s crazy. I was fishing on a boat when I was 19, here he is in the final of an ATP event.”

Novak Djokovic blamed mental and physical exhaustion after another desperate day for the Serbian at the Olympic Games meant he will leave Tokyo empty-handed.

The world number one lost 6-4 6-7 (6-8) 6-3 to Pablo Carreno Busta in the singles bronze medal match, smashing one racket against a net post and tossing another into the stands in gestures of frustration.

Djokovic then cited a shoulder injury as he pulled out of the mixed doubles third-place match. That decision meant Ash Barty and John Peers of Australia took bronze, with Djokovic and his unfortunate Serbia partner Nina Stojanovic finishing fourth.

"I am dealing with injuries. Not one, more than one," Djokovic said in an interview with Serbian media, according to tennismajors.com. "I hope that it won't stop me from going to the US Open, which is my next big goal.

"I feel bad for Nina because we did not fight for a medal in mixed, but my body said 'enough'. I have played under medications and abnormal pain and exhaustion."

The 34-year-old Djokovic said he had put his "very last source of energy" into the tournament and was satisfied with his effort, with a semi-final defeat to Alexander Zverev on Friday having left him resigned to a battle for what by his standards was perceived as a consolation prize.

But Djokovic added: "I know I've not played well today, and yesterday in the second and third set.

"The exhaustion, both physical and mental, got to me and it's unfortunate that in the most important matches I just didn't deliver, but I gave it all."

Djokovic suggested the Paris Olympics in 2024 were a possible target, although he must be becoming sick of Olympic tennis by now, having only one bronze to show for four attempts to win gold.

He took bronze in 2008 but lost to Argentina's Juan Martin del Potro both in the third-place match at London 2012 and the first round at Rio 2016.

Now there is more Games agony to digest, as well as a need to reboot ahead of the upcoming North American hardcourt swing and that US Open campaign. His hopes of a calendar Golden Slam are over but a sweep of the grand slams remains a possibility, having already landed the Australian Open, French Open and Wimbledon titles.

"I've had some heart-breaking losses at the Olympics Games," Djokovic said. "I know that those losses have usually made me stronger. I know that I will bounce back. I will try to keep going for the Paris Olympic Games. I will fight for my country to win medals."

Carreno Busta's reaction was thoroughly refreshing, with the Spaniard jubilant to secure a medal.

He said: "This week has been a very harsh week mentally for me. When I saw Novak lost, and I had to play him for this medal, I had my doubts.

"But last night I slept like I haven't slept in Tokyo. I slept for nine hours straight, that was an advantage to me. I came on to the court today knowing Spain was behind me.

"This is even more incredible than winning other tournaments. I've won Davis Cup, I've gone far in other tournaments, but winning an Olympic medal is indescribable. Words fail me, I felt Spain rallying behind me. A bronze medal is a dream come true for me."

The knock-on effort of Djokovic's withdrawal from the mixed doubles meant Barty and Peers added to Australia's medals haul without having to step on court for the third-place play-off.

Barty insisted she and Peers were worthy bronze medallists, saying: "It's incredible. It's unique circumstances and heartbreaking for Team Serbia not to get out on court.

"But for Johnny and I this is a dream come true for us. I feel like we've really deserved this one."

Novak Djokovic smashed one racket and threw another into the stands on the way to losing his bronze medal match at the Tokyo Olympics on Saturday.

Spain's Pablo Carreno Busta beat the world number one 6-4 6-7 (6-8) 6-3 to finish third in men's singles.

The shock result followed a day on from Djokovic's hopes of a Golden Slam being crushed by defeat to Alexander Zverev in the semi-finals.

In the wake of his singles third-place match setback, Djokovic also pulled out of the mixed doubles bronze match, citing a left shoulder injury.

Having already clinched the Australian Open, French Open and Wimbledon titles, Djokovic was eyeing gold at the Games before heading to the US Open in August, aiming to land the five biggest prizes in tennis in the same season.

That has never been achieved in a calendar year by a man, and Djokovic could not even manage a consolation prize from his singles mission in Japan. How serious his injury is now remains to be seen.

After levelling the singles match by taking the second-set tie-break, Djokovic boiled over in the first game of the decider, firstly when flinging his racket several rows back from the middle of the court after Carreno Busta put away a volley at the net.

Djokovic picked another racket from his bag but petulantly demolished that against a net post after dropping the third game to slide 3-0 behind, receiving a warning from the umpire for that violent outburst.

His anger may have been explained by injury or by his disappointment on Friday, when, as well as losing to Zverev, Djokovic and Serbian team-mate Nina Stojanovic were beaten in the semi-finals of the mixed doubles.

Djokovic would have had one final shot at a medal from his Tokyo trip to come later on Saturday, with the 34-year-old and Stojanovic due to face Ash Barty and John Peers in another match for bronze.

However, shortly after his singles exit, it was announced that Djokovic had pulled out of that match, handing Barty and Peers the medal.

Amid the anger and frustration exhibited on court by Djokovic, it was a banner day for Carreno Busta, as Spain celebrated another tennis medal, having won at least one in eight of the last nine Olympics.

The 30-year-old fell to the court in joy at the end of the two hours and 47 minutes it took him to defeat the world number one, his elation a sharp contrast to the emotions of his beaten opponent.

Fifth seed Taylor Fritz defeated compatriot fourth seed Reilly Opelka in a three-set epic in the quarter-finals at the Atlanta Open on Friday.

Fritz, who reached last week's Los Cabos Open semi-finals and is currently ranked 36th in the world, edged out Opelka 7-6 (7-5) 6-7 (3-7) 7-6 (7-4) in just under three hours.

The pair sent down a combined 48 aces, with Fritz claiming 25 of them, with all sets going to tiebreakers, with no breaks of serve.

Third seed Cameron Norrie was bundled out by Finnish world number 78 Emil Ruusuvuori in straight sets.

Norrie, who won last week's Los Cabos Open for his maiden ATP Tour title, was beaten 7-6 (7-1) 6-3 by the Finn.

Ruusuvuori will take on teenager Brandon Nakashima in the semi-finals, after the American – who was runner-up to Norrie in Los Cabos - beat Jordan Thompson 7-6 (7-5) 7-5.

Sixth seed John Isner sent down 30 aces as he got past Australian qualifier Chris O'Connell 7-6 (7-5) 6-7 (3-7) 6-4.

Alexander Zverev apologised to Novak Djokovic after ending the Serbian superstar's hopes of a glorious Golden Slam – but joked it was about time someone else landed a major tennis title.

In their Olympic Games semi-final, it seemed Djokovic was cruising through to the gold medal match when he surged a set and a break of serve ahead.

Incredibly, though, Zverev won 10 of 11 games from 3-2 behind in the second set to take the match 1-6 6-3 6-1 and set up a shot at Karen Khachanov in Sunday's final.

Djokovic swept to Australian Open, French Open and Wimbledon trophy success before heading to Japan for the Olympics, the fourth leg of a potential sweep of each of the year's majors and the Tokyo 2020 singles title.

He had spoken of it becoming closer to a reality, as he attempted to match Steffi Graf's achievement from 1988, when she followed triumphs at each of the slams by winning in the October 1 final at the Olympics, held in Seoul that year. Graf remains the only player to have pulled off the feat in the same year.

Zverev had other ideas, but he also had sympathy for Djokovic when they exchanged words at the net.

"I told him that he's the greatest of all time, and he will be," Zverev said.

"I know that he was chasing history, chasing the Golden Slam and chasing the Olympics, but in these kind of moments me and Novak are very close. Of course I'm happy that I've won, but at the end of the day I know how Novak feels.

"I feel sorry for Novak, but he's won 20 grand slams, 550 Masters Series or whatever, you can't have everything.

"He's the greatest player of all time, he will win the most grand slams out of anybody on tour, but I'm also happy that I'm in the final."

Victory at Wimbledon earlier in July took 34-year-old Djokovic to 20 grand slam titles, level with Roger Federer and Rafael Nadal for the most by a man in the history of tennis, and Zverev was lurching towards becoming his latest victim when their Tokyo tussle began in a one-sided manner.

"I was down a set and a break, so I needed to change something. I started playing much more aggressive," Zverev said. "I started to swing through the ball a little bit more, and I tried to dominate that way."

Zverev is assured of at least a silver medal now, while Djokovic faces a bronze medal play-off against Spain's Pablo Carreno Busta.

"It's an amazing feeling knowing that you're going to bring the medal back to your house, back home to Germany," Zverev said.

"It's incredible beating the best player in the world undoubtedly right now and in this season. It seemed it was impossible to beat him at this event, so I’m very happy right now. But yet there's still one match to go."