Marin Cilic won his 20th title on the ATP Tour by battling past Taylor Fritz in the final of the St Petersburg Open on Sunday.

A decade on from lifting the trophy for the first time, Cilic outlasted his American opponent 7-6 (7-3) 4-6 6-4 in a gruelling contest lasting two hours and 23 minutes.

It took the Croatian to four tournament wins on Russian soil, the 33-year-old having also twice won the crown in Moscow, while it also saw him join truly elite company in the men's game.

Only five other active male players have won 20 or more titles: Roger Federer (103), Rafael Nadal (88), Novak Djokovic (85), Andy Murray (46) and Juan Martin del Potro (22).

"Coming 10 years after winning in 2011 and also winning now, it's so, so special for me, especially this part of my career," said Cilic. "Having now a family, two sons, I want to absolutely dedicate this trophy to my newborn son."

Cilic had the chance to reach 20 titles last week in Moscow, only to lose in straight sets to home favourite Aslan Karatsev.

With 12 aces and 78 per cent of first-serve points won, the former world number three was aggressive in his efforts to make sure this was not another missed opportunity.

After taking the first set, Cilic twice led by a break in the second but allowed Fritz to level the contest and keep alive his hopes of a first title since Eastbourne two years ago.

Fritz went a break ahead in the decider but Cilic battled back to level at 4-4. A double fault from the world number 28 gave Cilic the chance to serve out the contest, and he duly obliged, converting his first match point with a volley.

Alexander Zverev captured his fifth title of the season at the Vienna Open after defeating Frances Tiafoe 7-5 6-4 in Sunday's final.

The Olympic champion claimed more silverware in Vienna, having also triumphed in Madrid, Cincinnati and Acapulco in 2021.

The world number four, who claimed his 300th tour-level career win against Alex de Minaur earlier in the week, is now 25-2 since exiting the fourth round at Wimbledon in July.

Tiafoe was appearing in his first ATP 500 final having defeated Stefanos Tsitsipas, Diego Schwartzman and Jannik Sinner along the way.

The world number 49 stuck with Zverev during a tight opening set, in which breaks were exchanged early on.

However, the German broke again in game 12 to draw first blood, and went on to hit eight aces during the second set.

Tiafoe produced an inspired turnaround from 5-2 down in the second set of his semi-final victory over Sinner.

But there was to be no fairytale finish for the American, who was seeking a second ATP title and first since the 2018 Delray Beach Open.

Zverev subsequently held his nerve to move level with Casper Ruud on five titles for the season, the most on the ATP Tour this year.

Frances Tiafoe will face Alexander Zverev in the Vienna Open final after storming back from the brink of defeat to beat Jannik Sinner.

Sinner was on the cusp of another final after his triumph in Antwerp last weekend when he led 5-2 in the second set, but qualifier Tiafoe roared back to win 3-6 7-5 6-2 on Saturday.

Italian Sinner had moved into the last qualifying position for the ATP Finals by beating Casper Ruud to reach the semi-finals in the Austrian capital and appeared destined to extending his winning run.

The 49-ranked Tiafoe had other ideas, winning five games in a row to force a deciding set.

Tiafoe maintained his momentum, breaking twice in the final set to complete an astonishing comeback and leave seventh seed Sinner shellshocked.

Second seed Zverev earlier marched into his fifth tour-level championship match of the season with a 6-3 6-3 defeat of Spanish teenager Carlos Alcaraz.

Zverev, who this week secured a 50th tour-level win of the season and a 300th of his career, needed only 68 minutes to knock Alcaraz out.

The German said: "I finally found my rhythm from the baseline because I was really struggling.

"I didn't feel the ball well and I spent some time on the court after the match yesterday and warmed up for an hour and a half before the match today and I finally got it."

Marin Cilic and Taylor Fritz will do battle in the St Petersburg Open final following contrasting wins over Botic Van de Zandschulp and Jan-Lennard Struff respectively.

Cilic did not face a break point in a 6-3 6-3 victory over Dutchman van de Zandschulp, while fought back to defeat Jan-Lennard Struff 5-7 6-1 6-3.

Top seed Andrey Rublev crashed out of the St Petersburg Open after a straight-sets defeat by Dutch qualifier Botic van de Zandschulp in the quarter-finals.

A recent US Open quarter-finalist, world number 69 Van de Zandschlup claimed the first top-10 win of his career on Friday.

The 26-year-old also advanced to the semi-finals of an ATP event for the first time.

There, he will face 2011 champion Marin Cilic, who was a 6-4 3-6 6-3 winner over third seed Roberto Bautista Agut.

Denis Shapovalov was another big-name casualty in Russia, the second seed going down 4-6 3-6 against world number 53 Jan-Lennard Struff.

Although, there were better fortunes for fifth seed Taylor Fritz. The Indian Wells semi-finalist is yet to drop a set this week after prevailing 6-4 6-2 against John Millman.

Elsewhere, Matteo Berrettini was the victim of another upset at the Erste Bank Open in Vienna.

The Wimbledon champion went down 1-6 7-6 (7-2) 6-7 (5-7) following a two-hour 40-minute battle with Carlos Alcaraz.

Reaching his first ATP 500 semi-final, the Spanish teenager claimed another big scalp having defeated Stefanos Tsitsipas at Flushing Meadows last month.

However, second seed Alexander Zverev remained on course for a fifth title of the season, as he claimed his 300th tour-level win.

The Australian Open finalist beat Felix Auger-Aliassime 6-4 3-6 6-3, avenging his fourth-round defeat by the Canadian at Wimbledon in July.

Alexander Zverev and Jannik Sinner moved into the quarter-finals of the Vienna Open on Thursday. 

Italian Sinner beat home hopeful Dennis Novak 6-4 6-2 to set up a tantalising showdown with Casper Ruud. 

Should he win that match, Sinner will move ahead of Hubert Hurkacz in the race to qualify for this year's ATP Finals, while victory for Ruud will strengthen the Norwegian's standing. 

"It's going to be tricky," said Sinner. "He's in a better position than I am for the race. I think right now it's tough to say and tough to do because in the end you try always not to think about that, but in the end you think about that because you believe." 

Zverev was given a sterner test, eventually overcoming Alex de Minaur 6-2 3-6 6-2 in one hour and 42 minutes. The Olympic gold medallist will now face Felix Auger-Aliassime, who fought back from a set down to defeat Cameron Norrie. 

Frances Tiafoe came from a set down to upset top seed Stefanos Tsitsipas 3-6 6-3 6-4 and earn a quarter-final meeting with Diego Schwartzman after the Argentine overcame Gael Monfils 7-6 (7-5) 4-6 6-2. 

In St Petersburg, home favourite and sixth seed Karen Khachanov lost a gruelling three-set encounter to Marin Cilic. 

The Croatian, who won the title in Russia in his last appearance in 2011, was 4-1 down in the deciding set but took all five of the final games to triumph after two and a half hours on court. 

"Everybody is playing good. The tournament is great. It's the end of the season, so everybody wants to finish on a great level," said Cilic, who next faces Roberto Bautista Agut. "For me, it was great to play this kind of a match today." 

Another Russian, Aslan Karatsev, fell to John Millman 6-3 6-2. Taylor Fritz and Botic van de Zandschulp also progressed in straight sets. 

Jannik Sinner continued his push for an ATP Finals spot with a straight-sets defeat of Reilly Opelka in the first round of the Vienna Open, while Carlos Alcaraz exacted revenge on Andy Murray.

Sinner won his fourth title of what has been an outstanding season for the Italian in Antwerp last weekend and is only 110 points adrift of Hubert Hurkacz in the battle for the final place in the season-ending event in Turin.

Murray did the 20-year-old a favour by knocking Hurkacz out of the ATP 500 tournament in the Austrian capital on Monday and seventh seed Sinner eased to a 6-4 6-2 win over American Opelka two days later.

Sinner, who will overtake Pole Hurkacz if he reaches the semi-finals this week, won 93 per cent of points behind his first serve and did not face a break point in a resounding win and will play Dennis Novak in the second round.

Murray beat Alcaraz at Indian Wells this month, but the 18-year-old rising star from Spain turned the tables on the three-time grand slam champion with a 6-3 6-4 second-round win in Vienna.

Former world number one Murray struggled with his serve and was broken five times as he made an early exit.

Third seed Matteo Berrettini beat Nikoloz Basilashvili 6-7 (5-7) 6-2 6-3 to reach the third round, while Diego Schwartzman, Gael Monfils and Lorenzo Sonego moved into the second round.

In the St Petersburg Open, defending champion Andrey Rublev beat Ilya Ivashka 6-4 6-4 to reach the quarter-finals along with Denis Shapovalov, a 2-6 6-3 6-0 winner against Pablo Andujar.

Jan-Lennard Struff also moved into the last eight, with Karen Khachanov and Marin Cilic securing first-round wins.

Novak Djokovic's participation at the Australian Open has been cast in doubt once again as Victorian premier Daniel Andrews indicated the state that hosts the event will not welcome those who are not fully vaccinated against COVID-19.

Australian minister for immigration Alex Hawke revealed last week Djokovic, who has won a record nine titles at Melbourne Park, could only take part in January's tournament if he has received both jabs.

But the 20-time grand slam winner, who has declined to reveal his vaccination status, was given hope of competing after prime minister Scott Morrison said there would be exemptions should the state of Victoria – which includes Melbourne – agree.

However, responding to those comments on Wednesday, Andrews made clear his government would not apply for any sort of dispensation as it would not be fair on the other players and spectators who are fully vaccinated.

"The federal government manages the border and to the extent that anything the federal government says on this is clear, because their position has gone 180 from what the immigration minister said," Andrews said.

"What I'm making equally clear on behalf of every vaccinated Victorian who is doing the right thing... the only fair thing to do is to be very clear with every Victorian, my government will not be applying for an exemption for any unvaccinated player."

Recent reports suggested that over a third of professional players on the ATP Tour remain unvaccinated ahead of the 2022 Australian Open, which runs for two weeks from January 17.

Andrews added: "I'm not going to ask and actually require people sitting in the grandstand, people working at the event, to be vaccinated while players aren't.

"So we're not going to be applying for an exemption, so the issue is basically resolved."

Andrews also stated the same approach will be taken when Formula One returns to Melbourne in April next year.

Speaking last week, world number one Djokovic – who beat Daniil Medvedev earlier this year to win his latest Australian Open crown – told Serbian newspaper Blic: "Things being as they are, I still don't know if I will go to Melbourne.

"I will not reveal my status whether I have been vaccinated or not, it is a private matter and an inappropriate inquiry.

"Of course I want to go, Australia is my most successful grand slam tournament. I want to compete, I love this sport and I am still motivated.

"I am following the situation regarding the Australian Open... I believe there will be a lot of restrictions just like this year, but I doubt there will be too many changes."

Stefanos Tsitsipas survived a minor scare in the first round of the Vienna Open before ultimately going on to beat Grigor Dimitrov in straight sets.

It was neck-and-neck for much of the first set, with neither player losing serve, but Dimitrov appeared to have the upper hand when he got to 6-3 in the tiebreak.

Tsitsipas recovered emphatically, however, the top seed reeling off five successive points to win the set and he went on to book his spot in the second round thanks to a 7-6 (8-6) 6-4 victory. He will face Frances Tiafoe next.

"Grigor is a tough competitor and the level of tennis was really high today. We both competed at our highest [level]," Tsitsipas said afterwards.

"That was great tennis that was being played during the entire match and I got really close to my [best] offence and showed incredible attitude [in] the last game, which was the one that was the most crucial of the match."

Alexander Zverev, seeded second, was also made to work in his first-round clash with Filip Krajinovic.

While he blew the Serbian away in the first set, he found himself 5-2 down in the second as Krajinovic looked set to level the match, but Zverev scrapped his way back and then served to love to win 6-2 7-5. Alex de Minaur awaits the German.

Casper Ruud and Felix Auger-Aliassime, seeded fourth and sixth respectively, were two other high-ranking players who also secured progression, boosting their ATP Finals qualification hopes.

Ruud was pushed hard by Lloyd Harris but eventually outfought the South African in his 7-5 7-6 (7-2) win, while Auger-Alisassime made fairly light work of Ricardas Berankis 6-3 6-2.

Indian Wells champion Cameron Norrie, who is also in with a shot of reaching the ATP Finals, beat Marton Fucsovics 7-6 (7-4) 6-1 to set up a meeting with Auger-Aliassime – the British number one is 11th in the FedEx ATP Race to Turin rankings, one above his next opponent.

At the St Petersburg Open, fifth-seed Taylor Fritz was the highest-ranked player in action and he needed a little over an hour to beat Emil Ruusuvuori 6-1 6-4.

Pablo Andujar defeated Federico Delbonis to set up a meeting with second seed Denis Shapovalov, while Ilya Ivashka beat Laslo Djere for the right to tussle with tournament favourite Andrey Rublev next.

The top two seeds will do battle in the European Open final following commanding victories for Jannik Sinner and Diego Schwartzman on Saturday.

Sinner, the number one seed, moved into his fifth final of the season with a 6-2 6-2 defeat of Lloyd Harris in Antwerp.

The 20-year-old Italian, who is in the hunt for a maiden ATP Finals spot, dominated Harris from the baseline and saved all three break points he faced.

There were only 16 unforced errors from the racket of the composed Sinner, who will be bidding to win his fourth title of the season on Sunday.

Sinner said: "I am very happy to be in the final. I just love playing here and love playing indoors, so hopefully I can play a great match again tomorrow."

Schwartzman ended 20-year-old American Jenson Brooksby's impressive run with a 6-4 6-0 victory.

The Argentinian struck 15 winners and secured five breaks as he marched into the championship match.

Russian second seed Aslan Karatsev will come up against Marin Cilic in his first final on home soil after knocking compatriot Karen Khachanov out of the Kremlin Cup.

Karatsev beat Khachanov 7-6 (9-7) 6-1, while sixth seed Cilic – a two-time winner of the tournament in Moscow – got the better of Ricardas Berankis 6-3 6-4.

Andrey Rublev's hopes of retaining the Kremlin Cup on home soil in Moscow are over after he suffered a surprise defeat to Adrian Mannarino.

Mannarino ultimately came out on top 5-7 7-6 (7-4) 6-3 but he had to do it the hard way, coming back from match point down to set up a meeting with Ricardas Berankis, who lost only tree games in a 6-2 6-1 win over Federico Coria.

Rublev was never cruising, however, as Mannarino only just fell short in the first set and then took the second to a tiebreak, which he eventually won to turn the tide.

The 33-year-old, who lost to Rublev in the final two years ago, took things up a notch in the decider and did not face a single break point as he got the job done despite the home support.

"The crowd was cheering for Andrey but that is normal," he said after reaching the quarter-finals. "I was happy to play with such a nice atmosphere.

"We have been playing for a long time without fans, so it is nice to enjoy a stadium with crowds now."

Second-seed Aslan Karatsev is now the favourite in Moscow, and the Russian had few issues in a 6-4 6-3 defeat of Egor Gerasimov.

Karatsev will face Gilles Simon next following his 6-3 6-2 win over Mackenzie McDonald, while Pedro Martinez won 6-3 6-4 over Filip Krajinovic to book his passage.

At the European Open in Antwerp, Andy Murray's hopes of winning a first ATP Tour title in two years were ended by number two seed Diego Schwartzman.

The Argentinian trailed 4-1 in the first set but fought back to win 6-4 7-6 (8-6) in his first ever match against the Scot, whose last tour triumph was in Antwerp.

In his on-court interview, Schwartzman said: "It was a pleasure to play against Andy. We had not played before and he is coming back and every week he is playing better and moving better.

"I have a lot of respect because when I grew up playing tennis, I was watching Roger [Federer], Rafa [Nadal], Andy and Novak [Djokovic] and right now playing against him is a pleasure for me."

Seventh seed Lloyd Harris is also into the quarter-finals after a straightforward 6-2 6-3 victory over Jan-Lennard Struff in just 72 minutes, Marton Fucsovics came through 6-7 (5-7) 6-3 6-1 against Roberto Bautista Agut and Brandon Nakashima edged past Henri Laaksonen 7-6 (7-5) 6-7 (5-7) 6-3 to earn the right to face Schwartzman.

Daniil Medvedev says it is "logical" that the age of Rafael Nadal, Roger Federer and Novak Djokovic has made it easier to play against the 'Big Three' of men's tennis.

For so long, that triumvirate has dominated the ATP Tour, with Djokovic winning three of the four grand slams on offer in 2021 – denied only a clean sweep by Medvedev at the US Open.

Nadal has struggled with a foot injury for most of the year, though, a problem that saw him miss out on Wimbledon, the Tokyo Olympics and Flushing Meadows after losing to Djokovic in the French Open semi-finals.

Federer, similarly, has missed large parts of the 2021 term after requiring a third operation on his right knee in the space of 18 months.

Medvedev accepted Djokovic had proven "he can beat everyone" in another sensational season, but – while heaping praise on a golden generation of tennis – the Russian said it is natural that time has caught up with all three.

"It would be silly to deny it…[it's] logical," he told reporters about the prospect of younger players replacing the greats at the top of the men's game.

"It is something natural. It is not something that I decide. It is evidence. They get old and now it is easier to play against them."

Medvedev only dropped one set across the entire tournament en route to securing a maiden slam at the US Open.

However, Medvedev was reticent to say the shift in power has taken place.

"I do not dare to speak of a generational change in tennis," Medvedev said.

"It is the best generation in the history of tennis.

"Nobody can come close to the results they have achieved.

"We all want to defeat that troika. And they don't want to lose either."

Medvedev then focused on his own progress, having pulled out of the Kremlin Cup citing injury problems from an early exit in Indian Wells.

"I want to win more Grand Slams," he continued. "I also want to be number one and be at the top of tennis for many years. But you can't win every tournament, it's impossible."

Karen Khachanov revealed he felt the nerves of playing in front of a home crowd as he marked his return to Moscow with a comeback victory in the Kremlin Cup.

Handed a bye for the opening round, third seed Khachanov made home advantage count in his first match as the Russian defeated Australian world number 50 James Duckworth 3-6 6-3 6-1 to reach the quarter-finals on Wednesday.

Khachanov – Olympic Games silver medallist, who triumphed in Moscow three years ago – will face another Australian, John Millman after he defeated Illya Marchenko 6-1 5-7 6-4 at the ATP 250 tournament.

"It always feel great to be back in your hometown, in a tournament where everything started for me as a professional tennis player," world number 31 Khachanov said.

"[This is] where I won my first matches, I got my first points. Since then, it's always great to come back home to see a crowd, [get] home support. It's also little bit more nervous, of course. But I always enjoy playing at home."

Khachanov was joined in going through to the last eight by sixth seed Marin Cilic – a two-time Kremlin Cup champion who overcame Tommy Paul 7-5 6-3.

At the European Open in Antwerp, top seed Jannik Sinner downed fellow Italian youngster Lorenzo Musetti 7-5 6-2 in the duo's first meeting as professionals.

"We never practice together, so it was kind of a new match," Sinner said post-match.

"He's an incredible player. He's a very great talent, so I hope we will have a couple of more matches on the pro tour, and obviously I'm very happy about my level today."

Arthur Rinderknech upstaged eighth seed Dusan Lajovic 6-3 4-6 6-2 to set up a quarter-final tie with Sinner, who is in the hunt for a fourth title of the season and a place at next month's ATP Finals in Turin.

Another emerging talent, qualifier Jenson Brooksby, bettered US Open quarter-finalist Botic van de Zandschulp 6-2 6-0, with Alejandro Davidovich Fokina sending third seed Cristian Garin packing 4-6 6-3 6-3.

Australian Minister for Immigration Alex Hawke insists Novak Djokovic would need to be fully vaccinated against COVID-19 in order to enter the country and defend his Australian Open title.

The Serbian is the top-ranked player in the world and could move clear of Rafael Nadal and Roger Federer for grand slam titles if he can claim his 21st at Melbourne Park.

However, Hawke's comments regarding vaccination requirements cast doubts over Djokovic's participation, with the 34-year-old previously declining to reveal his vaccination status.

"You'll need to be double vaccinated to visit Australia," Hawke said to Australian Broadcasting Corporation radio. "That's a universal application, not just to tennis players. I mean that every visitor to Australia will need to be double vaccinated.

"I don't have a message to Novak. I have a message to everybody that wishes to visit Australia. He'll need to be double vaccinated."

With recent reports suggesting that over a third of professional players have yet to be fully vaccinated, a significant number could be denied the chance to play in the opening grand slam of 2022.

The men's ATP and women's WTA tours have attempted to encourage players with reservations to get the vaccine, and Tennis Australia explained that it was working with government authorities regarding conditions for the tournament.

"Our understanding is that the details around international visitors entering the country are yet to be decided and we hope to have more information soon," Tennis Australia said in a statement.

Australia's health minister Greg Hunt defended the ruling, explaining that the decision had been taken with the safety of the country's citizens in mind.

"The [rules] apply to everyone without fear or favour," Hunt said. "It doesn't matter whether you are number one in the world or you are anything else."

Djokovic, who withdrew from the Indian Wells Masters this month, has won nine of his majors at the Australian Open.

World number one Novak Djokovic is unsure if he will defend his Australian Open crown in Melbourne next year due to vaccination requirements.

The state of Victoria, where the year's opening grand slam takes place at Melbourne Park, has introduced a vaccine mandate for professional athletes and across most industries amid the coronavirus pandemic.

The 2021 Australian Open went ahead, albeit in February instead of January, and without fans for most of the tournament following a snap lockdown of Melbourne due to COVID-19.

Djokovic was among the players critical of the conditions athletes endured prior to this year's Australian Open, with strict quarantine measures introduced, though the nine-time champion cast doubt over his participation.

"Things being as they are, I still don't know if I will go to Melbourne," 20-time major winner Djokovic told Blic. "I will not reveal my status whether I have been vaccinated or not, it is a private matter and an inappropriate inquiry.

"People go too far these days in taking the liberty to ask questions and judge a person. Whatever you say 'yes, no, maybe, I am thinking about it', they will take advantage."

"Of course I want to go, Australia is my most successful grand slam tournament. I want to compete. I love this sport and I am still motivated," said Djokovic, who has not played since losing in the US Open final, having withdrew from the Indian Wells Masters.

"I am following the situation regarding the Australian Open and I understand the final decision [on COVID-related restrictions] will be made in two weeks. I believe there will be a lot of restrictions just like this year, but I doubt there will be too many changes.

"My manager, who is in contact with the Australian Tennis Federation, tells me they are trying to improve the conditions for everyone, both for those who have been vaccinated and those who have not."

Victoria state premier Daniel Andrews responded to Djokovic's comments on Tuesday, saying it was unlikely unvaccinated players would be granted a visa to travel to Australia.

"I don't think an unvaccinated tennis player is going to get a visa to come into this country and if they did get a visa they'd probably have to quarantine for a couple of weeks," Andrews told reporters.

Cameron Norrie became the first British player to win the ATP Indian Wells Masters after fighting back to beat Nikoloz Basilashvili in an historic three-set victory.

Norrie dropped the opening set before rallying to a memorable 3-6 6-4 6-1 triumph for the biggest title of his career on Sunday.

The world number 26 became the first man ranked outside the top 25 to win in the Californian desert since Ivan Ljubicic in 2010 as he moved 10th in the Race to Turin, with the ATP Finals taking place next month.

"It means so much to me, my biggest title. I'm so happy. I can't even describe it right now," 21st seed Norrie said in his on-court interview. "It was a strange match today but absolutely massive for me and my team.

"I can't really believe it. If you’d have told me I'd have won before the tournament started I wouldn’t have believed you, so it's amazing."

Basilashvili – the 29th seed – was down an early break at 3-1 before turning the first set on its ear by reeling off five straight games to seize control.

It was the first time Norrie had dropped an opening set at this year's tournament and it was looking ominous when the new British number one surrendered another break at 2-1 at the start of the second.

But a run of eight consecutive points fuelled Norrie, who broke to level the match against Basilashvili.

While Basilashvili had his chances, Norrie used his defensive skills to outlast the Georgian after almost two hours on court.

Norrie, who boasts a 47-20 record in 2021, added: "I've been really enjoying my tennis and been enjoying being out on court and competing in the big moments.

"I'm just really pleased with how I handled the occasion. I think I'm doing a lot better with that this year. I lost a lot of those finals, so it's nice to get the big one today."