Novak Djokovic's hopes of becoming the first man to win a calendar Golden Slam were crushed by a semi-final defeat to Alexander Zverev at the Tokyo Olympics.

From a set and a break up, Djokovic dropped a staggering eight games in a row on his way to a 1-6 6-3 6-1 loss.

The 34-year-old Serbian had been unsure about coming to the Games but was swayed by the pride he takes in representing his country and the tantalising opportunity to add a gold medal to a potential clean sweep of the grand slams.

He has already won the Australian Open, French Open and Wimbledon, and will head to the US Open in August as a heavy favourite regardless of this setback.

But this was just not Djokovic's day, despite him making a whirlwind start and dominating until the point he broke to lead 3-2 in the second set. From there, Zverev seized control. 

When Djokovic volleyed into the net to make it 3-3, dropping serve for the first time in the match, it looked like just a minor stumble.

Yet suddenly he was struggling for form and did not win another game until he was already 4-0 behind in the deciding set.

Zverev was playing blindingly brilliant tennis and was proving obdurate too, saving four break points in the second game of the third set.

He clinched victory with a blazing backhand winner, and goes on to face Russian Karen Khachanov in the final.

The result means Djokovic, like Roger Federer, seems fated never to win the Olympic singles gold medal. And it leaves Steffi Graf as the only player to ever win a calendar Golden Slam, having done so in 1988 when she added the Seoul Olympics title to her haul of majors.

Russian Olympic Committee's Khachanov fended off Spain's Pablo Carreno Busta with some ease in the first semi-final, with the world number 25 sweeping to a 6-3 6-3 victory in an hour and 19 minutes.

He won 26 of 28 points on first serve to freeze out Carreno Busta, who had only one break point all match and could not take that opportunity.

"It's just a pure happiness, a pleasure to be here to live those moments, these kind of memories will stay forever," Khachanov said.

Khachanov delivered a rock solid display, with his serve and forehand at their best, barely giving his opponent a sniff of an opportunity as the Moscow-born 25-year-old established a firm grip.

"That's the way I prepared, against every opponent you play a little bit differently," Khachanov said. "The final will be another story, another match, another day. I hope it will be the same."

Carreno Busta, who will face Djokovic for the bronze medal, said: "It was not the best match I have played, but Karen was unbelievable today, playing very aggressive and serving really good."

Cameron Norrie breezed past Nick Kyrgios and John Isner downed countryman Jack Sock to advance to the quarter-finals at the Atlanta Open on Thursday. 

The third-seeded Norrie defeated Kyrgios 6-1 6-4 in less than an hour as he tries for a second consecutive title after collecting his first ATP Tour championship at Los Cabos last week. 

The former University of Georgia star Isner, a five-time champion in Atlanta, beat Sock 7-6 (8-6) 6-4 despite serving only 13 aces after hitting 36 in his previous match. 

Second-seeded Jannik Sinner fell 7-6 (9-7) 6-4 to Australian qualifier Christopher O'Connell, leaving the tournament without its top two seeds after Milos Raonic was upset by Brandon Nakashima on Wednesday. 

O'Connell, ranked 132 in the world to Sinner's 23, had not won a main-draw match this year before defeating Denis Kudla in the opening round and will now face Isner in the quarters.

Norrie will meet Emil Ruusuvuori, who advanced when Benoit Paire retired down 3-0 in the third set after the pair had split the first two sets 4-6 6-4. 

Ruusuvuori is into his second career ATP quarter-final after making it to the semis at Nur-Sultan last year. 

Top seed Milos Raonic saw his stay at the Atlanta Open come to an early end with a three-set defeat to American Brandon Nakashima on Wednesday. 

The 19-year-old Californian Nakashima, ranked 115th in the world, prevailed 5-7 6-3 7-6 (7-4) over world number 22 Raonic in their first meeting. 

The big-serving Canadian had 10 double faults to go with his 27 aces, while Nakashima did not commit one double fault and managed 13 aces. 

It was the latest in a string of recent wins over established players for Nakashima, who stunned Sam Querrey and John Isner last week at Los Cabos before falling to Cameron Norrie in the final. 

Nakashima next faces Australia's Jordan Thompson, who beat Peter Gojowczyk of Germany 7-6 (8-6) 6-4 to reach the quarter-finals. 

In the other half of the draw, fifth seed Taylor Fritz rallied to defeat fellow American Steve Johnson 6-7 (5-7) 6-4 6-1 in a rematch of a quarter-final at Los Cabos, also won by Fritz. 

Fritz will face countryman Reilly Opelka, who outlasted Bjorn Fratangelo 7-6 (7-3) 7-6 (7-4). The 6-foot-11 Opelka pounded 25 aces to just three double faults, winning 84 per cent of his service points. 

World number 16 Roberto Bautista Agut crashed out in the second round as the other favourites for the Generali Open battled through.

Bautista Agut lost the first set to Spanish compatriot Pablo Martinez in Kitzbuhel, Austria, and could not complete a comeback despite forcing a decider as he lost 6-4 6-7 (5-7) 7-5.

Top seed Casper Ruud, who won last week in Gstaad to claim his third ATP Tour crown of 2021 and fourth in total, came from 4-2 down in the first set to win 7-5 5-7 6-4 against Mario Vilella Martinez.

Third seed Filip Krajinovic survived a second-set scare to beat Carlos Taberner 6-3 2-6 6-4.

Arthur Rinderknech, who dispatched of fifth seed Federico Delbonis on Tuesday, awaits Krajinovic in the next round.

The other clash on Wednesday saw Sweden's Mikael Ymer cruise past home favourite Alexander Erler 6-2 6-3 to secure a quarter-final berth.

Ymer will now face the thankless task of challenging 22-year-old Ruud for a spot in the semi-finals.

Sixth seed John Isner sent down an Atlanta Open joint record 36 aces as he overcame countryman Jeffrey John Wolf in three sets on Tuesday.

World number 35 Isner was dominant on his first serve, winning 6-4 6-7 (3-7) 6-4 to claim a spot in the second round where he will face Jack Sock who beat Ricardas Berankis in three.

Isner's 36 aces equaled the previous Atlanta Open record set by Sam Querrey on Monday in his three-set win over Peter Gojowczyk.

American fifth seed Taylor Fritz also progressed on Tuesday with a 6-3 6-4 victory over Russian Evgeny Donskoy.

French seventh seed Benoit Paire got past Japan's Yasutaka Uchiyama 7-5 6-7 (2-7) 6-4, while enigmatic Australian Nick Kyrgios beat South African Kevin Anderson 7-6 (7-4) 6-3.

Teenage American talent Brandon Nakashima knocked out Trent Bryde, while Australian Chris O'Connell beat Denis Kudla and Finland's Emil Ruusuvuori got past Mackenzie McDonald 7-6 (7-3) 7-5.

Nakashima, who got a special exemption entry into the Atlanta Open, next takes on top seed Milos Raonic.

Federico Delbonis and Laslo Djere crashed out of the Generali Open in the first round on Tuesday.

Fifth seed Delbonis and sixth seed Djere were the highest-ranked players in action on day two, but both fell to surprise defeats.

Delbonis, a recent semi-finalist at the Hamburg European Open, was convincingly beaten 6-2 6-4 by Arthur Rinderknech.

Frenchman Rinderknech fired down 10 aces and was only broken once in the contest.

Djere, meanwhile, put up more of a fight but ultimately succumbed to Daniel Altmaier, who won 4-6 6-3 6-3 in a battle lasting two hours and 10 minutes.

World number 50 Djere had also made the semi-finals in Hamburg and followed that up with another run to the last four at the Swiss Open Gstaad.

But his run of form came to an end as German Altmaier – who himself enjoyed a semi-final run in Croatia last week – booked a last-16 tie meeting with Marco Cecchinato after breaking Djere on six occasions.

Cecchinato had beaten Radu Albot in straight sets to reach the next round while qualifier Jozef Kovalik saw off Jaume Munar with a 6-4 6-4 victory.

The top four seeds, including number one Casper Ruud, will begin their campaigns at the ATP 250 event in Kitzbuhel on Wednesday.

Andy Murray said the quad injury that ruled him out of defending his men's singles title at Tokyo 2020 has not been troubling him in matches, but his withdrawal was due to concerns over results showing up in scans.

The Olympic champion in 2012 and 2016 opted to pull out of the singles prior to his scheduled first-round showdown against Felix Auger-Aliassime on Sunday. 

Instead, Murray decided to continue in the doubles alongside Joe Salisbury, where the pair made the quarter-finals on Tuesday by beating German duo Kevin Krawietz and Tim Puetz 6-2 7-6 (7-2).

It means Murray remains on track for a third straight Olympic gold and he offered a bit of insight behind his decision not to play in the singles.

"I've had a bit of an issue with my quad," he said.

"It's actually not really been causing me many issues on the court like in practice or anything, there was just some stuff showing on the scans that made all the medical team a bit wary about it.

"I said to Joe that if he picked me to play doubles with him then I'd prioritise the doubles over the singles if I had any physical issues and that was the case. 

"It's disappointing for me because I do feel like I've been playing well, and I've loved the Olympics and I would've liked the opportunity to defend my title. 

"But that wasn't to be and now all the focus and energy goes towards the doubles and to try our best to get a medal there."

Marin Cilic and Ivan Dodig await Murray and Salisbury in the last eight.

South African eighth-seed Lloyd Harris was bundled out by Australian Jordan Thompson 7-6 (7-2) 1-6 6-2 in a seesawing contest.

Thompson, who is ranked 63rd in the world, got the better of Harris, showing to grit to win in two hours and 30 minutes.

In the final match of the day, play was suspended due to rain and lightning after the first set with American Sam Querrey leading 6-4 against German Peter Gojowczyk.

Former world number 11 Querrey sent down 12 aces in the first set before the weather intervened.

The winner of Querrey and Gojowczyk will take on Thompson.

Steve Johnson defeated Alexei Popyrin 7-6 (7-4) 6-3, while Bjorn Fratangelo beat Italian Andreas Seppi 7-5 7-6 (7-5).

Pedro Martinez outlasted Lucas Pouille to set up a second-round clash with fellow Spaniard Roberto Bautista Agut at the Generali Open.

World number 97 Martinez came through Monday's longest match, which lasted two hours and 43 minutes, as a 6-3 6-7 (4-7) 6-4 winner.

Mikael Ymer, who reached the quarter-finals of last week's Swiss Open Gstaad, is also through to the second round in Austria after battling past Pablo Cuevas 6-3 7-5.

Qualifier Jozef Kovalik will face Jiri Vesely for a place in the quarter-finals, meanwhile, after beating ninth seed Jaume Munar in straight sets.

Also through on Monday was lucky loser Carlos Taberner, who took advantage of his second chance by battling from a set down to overcome Thiago Seyboth Wild 3-6 6-3 6-2.

Carlos Alcaraz won his first of possibly many ATP Tour titles on Sunday with an emphatic victory over Richard Gasquet at the Croatia Open. 

The 6-2 6-2 win in Umag meant 18-year-old Alcaraz became the youngest tour-level champion since Kei Nishikori won at Delray Beach in 2008, also aged 18.

Despite his tender years, seventh seed Alcaraz was too hot to handle for Gasquet, the highly rated Spaniard cruising past his significantly more experienced opponent in one hour and 17 minutes.

"I had a lot of good moments in this tournament. I beat five great tennis players," Alcaraz told atptour.com.

"I think that I grew up a lot in this tournament and I keep a lot of experience from this tournament. It's going to be useful for the future."

Gasquet, who needed more than three hours to overcome Daniel Altmaier in the last four, was hoping to win his first tour title since 's-Hertogenbosch in 2018.

"It was tough for me to play [with] full intensity. I had a tough match yesterday. It was tough, and especially with a guy like Carlos, who is playing really fast with a lot of energy and spin," Gasquet said.

"He’s playing unbelievable. He’s only 18 and of course he has a great future and I just couldn’t play at his level and his intensity.

"That was the key of the match and he didn't lose a point. He played well, very solid. He's a great player."

Stefanos Tsitsipas is taking inspiration from the grandfather he has never met after battling through to round two of the men's singles at Tokyo 2020.

Alexander Zverev had an altogether more comfortable progression at Ariake Tennis Park, while home favourite Kei Nishikori earned an impressive upset win.

Felix Auger-Aliassime was unable to make the most of two-time defending champion Andy Murray's withdrawal, but it was a good day for Hubert Hurkacz. 

Here's the pick of the action from day two of the men's singles.

TSITSIPAS OUT TO EMULATE GRANDFATHER

Greek ace Tsitsipas, a French Open finalist this year, had to dig deep for a three-set win over Philipp Kohlschreiber.

Should he manage to win gold in the Japanese capital he would be matching the feat of his grandfather, who won the football competition representing the Soviet Union in 1956.

"I've never had the opportunity to meet him. But my mom told me stories of his career and how he got it," he said. "He kind of inspires me in a way. I know what kind of athlete he was, with all the achievements and all the trophies. I'm proud of him. 

"It's something good, a legacy that is being carried on in the family. I'm happy to be the next in the family to be competing at the Olympics."

ZVEREV LOVING OLYMPICS EXPERIENCE

Fourth seed Zverev coasted past Lu Yen-hsun 6-1 6-3 and spoke of how much he is enjoying being around other German athletes.

"Normally you don't have those guys around that much, you have your friends, of course you have people that are around you, but you don't sleep in the same room as them," he said.

"Yes it is very different but in a way very enjoyable. The Olympics are once every four years, and it’s five years now, so I think everybody is enjoying it and everyone is having the best time that they can."

Nishikori is playing at a fourth Olympics and upset fifth seed Andrey Rublev 6-3 6-4. For the 31-year-old the motivation is simple.

"It's [playing in Japan] something I always dreamed of when I was little," he said. 

"Especially now, with the Covid situation, if I can win as many as I can, I think it will bring better news, that's something I'm trying to do this week."

AUGER-ALIASSIME FAILS TO MAKE MOST OF MURRAY ABSENCE

Auger-Aliassime was scheduled to face Murray before the Team GB star pulled out with a quad issue and will instead focus on doubles.

The Canadian was felled by Murray's replacement Max Purcell, though, the Australian winning 6-4 7-6 (7-2).

Fellow seed Gael Monfils also crashed out but Hurkacz was a 6-2 6-4 victor over Luke Saville, while Diego Schwartzman also made it through.

Casper Ruud secured back-to-back titles by beating Hugo Gaston in straight sets to be crowned Swiss Open Gstaad champion on Sunday.

Ruud won the Nordea Open last weekend and added a fourth ATP Tour title of his career with a 6-3 6-2 defeat of Frenchman Gaston.

The Norwegian took his tally of tournament victories on clay this year to three, having also won the Geneva Open in May, and the 22-year-old will rise to ninth spot in the ATP Race To Turin on Monday.

Ruud saved nine of the 10 break points earned by first-time ATP Tour finalist Gaston, who only won 51 per cent of points behind his first serve.

Gaston, who had never been beyond the second round at an ATP Tour event before this week, was broken twice in each set as the 20-year-old was unable to halt Ruud's brilliant run on clay.

Left-hander Gaston broke straight back after Ruud went 3-1 up in the opening set but failed to back that up with a hold.

Ruud had as many as 16 break-point opportunities but got the job done in an hour and 34 minutes.

World number 30 Cameron Norrie has claimed his maiden ATP title after defeating Brandon Nakashima in straight sets in the final of the Los Cabos Open on Saturday.

British top seed Norrie triumphed 6-2 6-2 in one hour and 23 minutes as he marks another milestone in his career-best season.

Norrie, 25, now has the most wins on the ATP tour this year, going past world number one Novak Djokovic with 35.

The 19-year-old American could not handle Norrie's first serve, while the Briton converted five of his eight break points.

Norrie served for the match with Nakashima pressing him in a tight final game but the top seed rounded out a breakthrough win.

Norrie won 21 of 29 points on his first serve, going at 72 per cent, with Nakashima breaking him once.

Hugo Gaston is one win away from securing his maiden ATP Tour title, but he must overcome in-form Casper Ruud at the Swiss Open Gstaad.

Gaston, ranked 155th in the world, booked his place in his first tour-level final with a 3-6 6-3 6-3 victory over Laslo Djere of Serbia on Saturday.

The 20-year-old had never advanced past the second round at an ATP Tour event before this week. He has now won three matches in a row in a third set.

"I tried to take more time with my game," Gaston said. "I made a lot of mistakes in the first set, so I tried to change my game. My serve was better in the second and third set.

"I am really happy to be in the final. It is going to be a good match, but I will try to enjoy [this] moment."

Ruud, meanwhile, enjoyed a rather more straightforward procession in his semi-final, with world number 249 Vit Kopriva little match for the 22-year-old Norwegian.

The world number 14 needed just 74 minutes to win 6-3 6-0, reaching his second final in as many weeks following his success in Bastad.

Ruud has won 23 clay-court matches already in 2021 and is in the hunt for his fourth career title.

Naomi Osaka lighting the Olympic cauldron will have helped increase the exposure for the sport of tennis around the world, according to Novak Djokovic.

Osaka was given the honour of carrying the torch on the short final leg at the Japan National Stadium before walking the steps to light the flame and end the opening ceremony.

The four-time grand slam champion is the face of the Tokyo Games in her home country, creating extra pressure on her shoulders as she bids to strike gold.

Djokovic appreciates the absences of legendary names Roger Federer and Rafael Nadal is a blow, but he believes Osaka's presence is crucial for tennis in the battle for media coverage at the Games.

"It cannot be better for our sport, you are representing yourself, your country but also your sport in the Olympic Games, you're trying to get the hype and the attention towards our sport as much as you possibly can, so we're all contributing to that in the Olympic Village," Djokovic said when asked about Osaka's role in Friday's ceremony.

"There's a lot of attention towards the tennis players which is great, from the other athletes which is very nice to see, very nice to experience.

"Obviously you don't have Roger or Rafa. They are big stars and legends of our sport, but still there's quite a lot of great athletes, top players. Naomi is a home favourite and a lot of eyes are on her.

"Being at home playing is a lot of pressure, but it's great for our sport just in general to see that there's a lot of attention towards it."

Djokovic was speaking after overcoming Hugo Dellien with ease in his opening outing in the men's singles tournament, the Serbian triumphing 6-2 6-2 in just over an hour.

The quick win allowed the recently crowned Wimbledon champion to avoid staying out for too long in the Tokyo heat, an issue that led to Daniil Medvedev calling for matches to be pushed back to later in the day, allowing players to compete during the evenings when the temperatures have dropped.

"I agree with him 100 per cent," Djokovic said of Medvedev's suggestion. "I actually asked as well.

"My team captain, Viktor Troicki, was speaking to the referee a couple of times. To be honest, I don't understand why they don't start matches at say 3pm.

"I've heard for tennis there is some kind of curfew they have to finish at midnight, but if that's the case, I've just finished the last match and it's not even 5pm, we still have seven hours to play. 

"They have lights on all the courts, they're going to make life much easier for all of us tennis players, I just don't understand why they don't move it. 

"It's actually for the television broadcasters even better, because the later you play, the better it is for the United States and the time zones in Europe.

"I don't know, maybe the ITF (International Tennis Federation) can give you a better answer to why they chose to be played in the middle of the day. I doubt they will change the decision, but we're hoping that they will."