Rafael Nadal succumbed to defending ATP Finals champion Alexander Zverev as the world number seven put in a clinical display to triumph 6-2 6-4.

Heading into this year's season finale with concerns over an abdominal problem, Nadal showed no signs of struggling with injury, but the world number one could still not handle Zverev on Monday.

While Novak Djokovic - who Nadal ousted at the top of the rankings last week - started in dominant fashion against Matteo Berrettini, Nadal suffered a similar fate to that of Roger Federer, who went down to Dominic Thiem in his opening match.

Three successive breaks ultimately did the damage for Zverev, as the 22-year-old claimed a maiden win over Nadal to make a statement of intent in London.

"It's great, everyone knows how much I've been struggling this season," Zverev, who will also face Stefanos Tsitsipas and Daniil Medvedev in Group Andre Agassi said. "This means so much, playing here again after winning my biggest title here last year."

Any concerns over Nadal's fitness were cast aside with the first serve of the match - a venomous 126mph effort at the start of a game the 33-year-old claimed with relative ease.

But having made Nadal work far harder to hold his next serve, Zverev, who utilised the speed of the court to his advantage throughout, made the first of three straight breaks to take control.

Last year's winner stepped up another level in game seven, using a challenge well after a fantastic down-the-line forehand had been called out as Nadal conceded serve once more.

Having served out a 35-minute first set, Zverev needed no second invitation to take a third break point and - after rallying back from two double faults - held a 2-0 lead in set two.

Nadal finally regained his composure, and after holding to end his losing streak, the 19-time grand slam champion sent a sublime backhand pass to spark the O2 crowd into life.

A backhand into the net saw Zverev squander a break point for the first time - Nadal taking advantage to hold serve and keep himself in contention.

Yet Zverev had victory in his sights, though he lost some control on his first serve to hand Nadal hope of a break.

An impeccable, ripping forehand put him on the cusp, however, with the German duly serving out a first win over his Spanish counterpart in six attempts.

Stefanos Tsitsipas has claimed his 7-6 (7-5) 6-4 win over Daniil Medvedev at the ATP Finals "means more than extra".

The pair have both enjoyed terrific campaigns in 2019 - while Medvedev has won four titles and made it to a further five finals, including in the US Open, 21-year-old Tsitsipas has cemented himself in the world's top 10.

However, it is no secret Tsitsipas and Medvedev do not see eye to eye, a rivalry which was established in March 2018 during their first meeting.

Medvedev came from behind to win, but was unhappy with his Greek opponent for a heated handshake, with the umpire forced to intervene.

Tsitsipas' triumph on Monday is his first in six attempts against Medvedev, who has won 29 of his last 34 matches and was in the hunt for a 60th tour-level win of 2019.

"It means more than extra," Tsitsipas told reporters after his win. "Our chemistry definitely isn't the best."

According to Tsitsipas, their rivalry began when Medvedev complained that the youngster did not hold his hand up to apologise for a rally winning shot hitting the net during their clash in Miami last year.

"He started telling me that what I do is unsportsmanlike. I tried not to pay attention, because I knew that it was something intentional, something that he wanted to pass to me," Tsitsipas added.

"Somehow it did affect me. I did get p***** and said what I said, which I do regret, but at the time I was very frustrated that things happened this way."

World number four Medvedev, meanwhile, acknowledged the better player won at the O2 Arena on Monday.

"He was better today, but I felt like I was missing some things." the Russian told reporters. 

"This frustrates me after. I do think it would frustrate me against any other opponent. Of course I wanted to make it an even bigger head-to-head, but it's the way it is."

Stefanos Tsitsipas produced a scintillating performance to beat rival Daniil Medvedev for the first time in a battle of two ATP Finals debutants at the O2 Arena.

Medvedev had won all five encounters with Tsitsipas before the Greek finally came out on top on Monday, winning 7-6 (7-5) 6-4.

Tsitsipas had a spring in his step from the start against an opponent whose game he has described as "boring" and did not face a solitary break point in the Group Andre Agassi opener.

Just the one break in the second set was enough for a fired-up Tsitsipas to seal a straight-sets victory over the fourth seed from Russia, who has been outstanding this year but faces a tough ask to reach the last four in London.

Tsitsipas started on the front foot and forced a break point in the second game, but Medvedev held to level at 1-1. 

That was the one break-point opportunity of the opening set, and it was Tsitsipas who then came out on top in a tight tie-break, punching a volley on the line for a third mini-break and letting out a roar as his opponent netted on the first set point.

There was no let-up with the high tempo in the second set as the two continued to trade blows from the baseline and demonstrate their prowess at the net.

Medvedev fended off two break points before holding for a 4-3 lead, but he gifted a relentless Tsitsipas a break point when he left a ball which landed in, and an errant backhand left him 5-4 down.

Powerful sixth seed Tsitsipas jumped for joy after serving out a match in which he won 89 per cent of points behind his first serve. 

Roger Federer blamed "first-round hiccups" for the loss to Dominic Thiem that leaves him at risk of an early exit from the ATP Finals.

Austrian Thiem completed a hat-trick of wins over the 20-time grand slam champion in 2019 with a 7-5 7-5 victory on the opening day in London.

Federer now faces a tough task to advance from Group Bjorn Borg, which also contains Novak Djokovic and Matteo Berrettini.

The 38-year-old committed 30 unenforced errors and won 11 of 22 points on his second serve in an uneven showing at the O2 Arena on Sunday.

"I thought he played well. The start definitely didn't help. That put me on the back foot a little bit," Federer said.

"But I recovered well and I thought the match was pretty even for a long period of time. I felt I had my chances. Didn't feel like I was outplayed or anything.

"Just maybe those first-round hiccups a little bit, not hitting your spots on the serve when you need to, getting into trouble early in the service games, which maybe doesn't happen later in the tournament."

Federer won the last of his six ATP Finals titles in 2011 and will have to be at his best to stay in contention for a seventh.

"It's a normal tournament from here on forward. I'm not allowed to lose anymore," the Swiss star said.

"That's how it is every week of the year for the past 20 years, so from that standpoint there is nothing new."

Both players are back in action on Tuesday as Federer faces tournament debutant Berrettini, who was no match for a dominant Djokovic.

The manner of the Serbian's 6-2 6-1 triumph and the tight schedule gives Thiem little time to savour his fifth career win over "probably the best player of all time".

"The system is pretty brutal, I would say, because I had an amazing victory," he said.

"I'm feeling great. I'm feeling now that I have a good chance to reach the semis, but if I lose on Tuesday, for example, everything is completely open again and I [would] have to worry about everything on the next match on Thursday.

"The only thing I can do is rest well [on Monday], practice well, and be fully focused again on Tuesday."

Dominic Thiem continued his dominance of Roger Federer this year with a straight-sets victory on the opening day of the ATP Finals.

Thiem had beaten the 20-time grand slam champion twice this season and completed the hat-trick with a 7-5 7-5 victory at the O2 Arena on Sunday.

The fifth-seeded Thiem has not reached the semi-finals in his three previous appearances in London, but made a confident start after Novak Djokovic beat Matteo Berrettini in the opening Group Bjorn Borg match.

Thiem served and returned superbly, improving his record against Federer to five victories and two defeats as he bids to end a fruitful 2019 by claiming a sixth title of the year.

Federer dominated the robin-round encounter between the two in this tournament last year, but was a break down after firing a forehand into the tramlines in the opening game.

The Austrian was pegged back at 2-2 when Federer forced an error with a ferocious backhand, yet the six-time champion was unable to cut out the errors and trailed 6-5 after Thiem pounced on his second serve.

Thiem served out the set with great composure and both players had a packed crowd purring as they showed their athleticism with classy winners at the net early in the second.

Federer fizzed a majestic backhand winner down the line and saved a break point before levelling at 2-2, while Thiem continued to pepper his veteran opponent with powerful, accurate ground strokes.

Thiem drew gasps with a scorching one-handed backhand winner and broke to love to lead 6-5 when Federer netted after a blistering return was arrowed at his feet.

Federer went down with a fight, but the third seed was beaten when he dumped a backhand into the net after seeing two points to get back on serve come and go.

Novak Djokovic made a superb start to his ATP Finals campaign with a dominant 6-2 6-1 triumph over tournament debutant Matteo Berrettini.

In the first match of this year's season finale, Djokovic – who is aiming to oust Rafael Nadal as world number one after he was dethroned by the Spaniard earlier this week – made a statement of intent in the Bjorn Borg group.

Five successive, clinical breaks of serve saw last year's runner up – a four-time champion at the O2 Arena – ease to an emphatic victory in just 63 minutes.

Berrettini, the last player to qualifier for the finals, walked towards the wrong exit after Djokovic's win was sealed on a first match point, and given the beating he had just taken, it was perhaps no surprise the Italian seemed slightly shocked.

"It feels great to be back in, earlier this year I had a great stay in London, played well at Wimbledon," Djokovic told BBC Sport.

"Over the years I've played really well in this arena. I've been enjoying it very much.

"It's not easy for Matteo, playing his first tour finals match, I knew he'd be a bit more nervous at the beginning, I tried to use my experience.

"I was fortunate to get the break midway through the first set. He's got big weapons with his serve and forehand but I managed to play really solid."

Djokovic beat Roger Federer in a Wimbledon final for the ages in his last appearance in London, and the Serbian was swiftly into his rhythm against world number eight Berrettini.

Having not dropped a point in the opening game, the 32-year-old forced Berrettini to deuce on his first serve before two breaks handed him the set.

Berrettini claimed a brilliant point in game two of the second set, managing to reach for a low backhand that Djokovic could only skew into the air, providing his opponent with a simple volley.

But it merely stalled Djokovic's process for a brief moment – Berrettini, who is renowned for his powerful serves, going 2-0 and a break down.

Another break for Djokovic followed, though Berrettini did capitalise on the only break point he was afforded.

It mattered little, however, Djokovic getting the better of his opponent's serve once more before rounding off the win with a controlled cross-court forehand.

Kristina Mladenovic and Caroline Garcia made up for their painful 2016 Fed Cup final defeat as the pair overcame Ashleigh Barty and Samantha Stosur to secure France's third title.

Barty was flawless as she drew Australia level on day one with a 6-0 6-0 triumph over Garcia, but the world number one failed to recapture her form on Saturday as the hosts went down 3-2.

WTA Finals champion Barty started well in Perth and looked to be on the verge of another dominant win when she went a set up against Mladenovic in her singles rubber, yet it was the world number 40 who proved the star of day two.

After a sensational 2-6 6-4 7-6 (7-1) comeback victory over Barty, Mladenovic starred alongside Garcia as France clinched the trophy with a 6-4 6-3 doubles win, three years on from the same pairing suffering defeat to the Czech Republic in a decider.

"There are lots of emotions, to share this with Caro," Mladenovic said after a superb, instinctive volley from Garcia had proved too much for Barty's partner Samantha Stosur to handle on France's third match point.

"Three years ago we failed on the last step of the tie and we just wanted to take revenge for ourselves. It's indescribable how we feel right now."

Ajla Tomljanovic had stepped up for Australia by overcoming Pauline Parmentier 6-4 7-5 in Saturday's second singles contest to set up the grandstand finish, before Barty and Stosur made a fine start in the decider with an immediate break.

However, Mladenovic and Garcia rallied to draw level at 2-2 and a second break handed them the opening set.

Successive concessions of serve from the Australians put France into a commanding position in the second set, with Barty and Stosur subsequently failing to take advantage of three break points in the next game.

France were in no mood to let victory slip out of their grasp and – after Australia clawed back two match points to hold serve – Garcia's reactions got the better of Stosur, sparking jubilant celebrations from Julien Benneteau's team.

"I'm the proudest man on the planet right now," France's captain said. "It's a dream for me. I'm going to have a lot of beers!"

Eighteen-year-old Italian Jannik Sinner claimed an incredible Next Gen Finals title with a 4-2 4-1 4-2 victory against top seed and world number 18 Alex De Minaur.

Sinner has burst onto the scene in Milan and was in no mood to let down an expectant crowd in the final on Saturday evening.

The teenager's power was simply too much for De Minaur, who could not turn early pressure in the first two sets into a breakthrough against his inexperienced opponent.

Sinner had to dig in for the first hold of the match, but the opener stayed on serve then until the sixth game.

Backed by raucous support, a stunning Sinner return teed up set point on De Minaur's serve, claimed in equally devastating fashion.

De Minaur went on the offensive again but once more found Sinner resolute, this time swift in offering a spectacular response with a second break to love of the contest in the next game as the Australian went long.

Sinner had to save break points in his subsequent two service games but saw out the set impressively, before going on the offensive again in the third.

The reeling De Minaur battled bravely but was broken in the third game as his second final defeat in two years, having lost to Stefanos Tsitsipas, moved ever closer.

More brutal Sinner serving got the job done in straight sets, and he accepted the acclaim of Italian fans revelling in this stunning sudden ascension.

World number one Ashleigh Barty crushed Caroline Garcia 6-0 6-0 to square the Fed Cup final at 1-1 at the end of play on Saturday.

Australia trailed France following the opening rubber but were back level just 56 minutes after Barty took to the court in Perth.

The WTA Finals champion sent down eight aces and hit 15 winners to set up a blockbuster showdown with Kristina Mladenovic, who earlier breezed past Ajla Tomljanovic.

Mladenovic broke serve in the opening game and never looked back in a convincing 6-1 6-1 victory, but will have a tougher time getting past an in-form Barty on Sunday.

"I couldn't have asked for a more perfect match," the Australian said after seeing off Garcia.

"I think that's probably the best tennis match I've ever played in my life and what a place to do it, this is incredible. I'm so happy to be back here in Perth. I started my year here, so it's a hell of a way to finish it off too."

Barty is due to partner veteran Sam Stosur in the doubles, with Mladenovic and Garcia to team up for France.

Alex de Minaur and home favourite Jannik Sinner will meet to decide this year's champion at the Next Gen ATP Finals after respective semi-final triumphs over Frances Tiafoe and Miomir Kecmanovic.

Defeated by Stefanos Tsitsipas in last year's final, top seed De Minaur was on scintillating form against his good friend Tiafoe in a 4-2 4-1 0-4 4-2 victory.

The world number 18 – who has three singles titles to his name this season – hardly had to break a sweat as he took complete control in the first two sets.

Heeding the in-game advice of his coach, Tiafoe put in "unconditional effort" at 2-0 down, managing to haul himself back into contention with a supreme display in the third set.

However, De Minaur ultimately had too much firepower and, after a winner teed up match point, a stray shot into the net from Tiafoe handed the 20-year-old a second successive final appearance.

"It's difficult, but easier in some ways [to play a friend]," De Minaur said afterwards. 

"It's easy because you know that no matter what happens on the court, you're still going to be the best of friends.

"In the same manner, you've got to remember that you are opponents, I've got to get in the zone, fired up and motivated, but it worked out."

Facing the Australian in Saturday's title decider will be a favourite of the Milan crowd – 18-year-old wild card Sinner, who rallied from a set down to defeat Kecmanovic 2-4 4-1 4-2 4-2.

Some nervy shots from the world number 95 allowed his Serbian opponent to take a foothold, but Sinner – buoyed on by voracious supporters – soon regained his composure.

Successive breaks of serve across the end of set two and start of set three gave Sinner an advantage he did not relinquish and although Kecmanovic clawed back three match points in the final set, he sent a forehand return out of play to seal the Italian's progression. 

Rafael Nadal is set on ending 2019 as world number one despite doubts over his fitness heading into the ATP Finals.

Nadal overtook Novak Djokovic atop the ATP rankings on Monday but is still not certain of retaining the position at the end of the year.

Djokovic and Nadal are in opposite groups in London, with the Spaniard to face Stefanos Tsitsipas, Alexander Zverev and in-form Daniil Medvedev – who he beat in the US Open final in a five-set epic.

The 33-year-old took to Twitter on Friday to provide an update on his fitness, stating he is "taking it day by day," after he was forced to withdraw from the Paris Masters due to an abdominal tear, though doubt remains as to whether he will be able to participate through the whole tournament in the English capital.

However, Nadal insisted he is still determined to cap off the year at the top of the rankings, should he stay fit.

"Of course I would love to be the year-end number one but I always said it was not my personal goal because I did not follow that [path]," Nadal said in a news conference at the O2.

"If I did, I would be flying to China after New York because I was in a positive position.

"But that doesn't mean I am renouncing to try to be number one. Not at all. I am fighting for it."

Nadal, whose injury has prevented him from practicing, is hopeful of being fully fit for Monday's encounter with defending ATP Finals champion Zverev.

"I'm excited to be here after a couple of years without being able to play. I need to see how things evolve every single day," he said.

"I have good hopes to be 100 per cent ready for Monday. I had been serving very well in Paris, I had good matches, so I am confident that I can be very competitive.

"Of course it's a tournament that you will face the top guys since the beginning so you need to be 100 per cent ready. 

"But I really hope I will be able to serve every single day a little better and my hope is to be on Sunday serving normal."

Roger Federer has stuck by his decision to withdraw from the inaugural ATP Cup in order to focus on rest and time with his family ahead of the 2020 season.

Federer was set to lead Switzerland at the new 24-team event, which will take place in Australia in January.

However, the 38-year-old confirmed at the end of October that he would not be taking part in the competition, with Switzerland subsequently withdrawn from contention as a result.

After skipping the Paris Masters, 20-time Grand Slam champion Federer will now look to finish his season on a high in the ATP Finals in London.

He faces world number five Dominic Thiem on Sunday, having lost in the semi-finals to eventual winner Alexander Zverev in 2018.

Ahead of the event, Federer explained he has no regrets about deciding to not push his body too much early on in 2020, allowing him to spend more time with his wife and children.

"Something had to give, that was the ATP Cup," Federer said on media day at the O2 Arena.

"I just felt I was going to be very happy to play but it wasn't that level of importance for me. That was that.

"Normally I don't take these kinds of decisions like this but I kind of got into that situation having to take a decision after Wimbledon. 

"With my wife we try to come up with a really good schedule for the kids. We wanted to be in the same place for a long time. I don't think it's contradictory at all."

Federer conceded he is taking something of a risk by not playing in a warm-up tournament before the Australian Open, but is confident his experience will allow him to cope with the physical demands of heading straight back into a grand slam.

"At the end of the day I think with age and experience I can be confident about what I do in training," Federer said. 

"I'll travel to Melbourne early to make sure I give myself the best chance to get ready. I believe I can be ready, I don't think I need a tonne of matches always, especially on the hard courts.

"Maybe I'm a bit more dependent on the draw earlier on in the Australian Open. The key is health."

After facing Thiem, Federer will also go up against Matteo Berrettini and Novak Djokovic – who was recently dethroned as world number one by Rafael Nadal – in the group stage in London.

Rafael Nadal revealed he has started serving "very slowly" and will "see how things go" as he delivered a mixed injury update ahead of the ATP Finals.

The US Open winner is in London ahead of the year-ending tournament as he recovers from an abdominal tear sustained last Saturday.

Nadal had to pull out of his Paris Masters semi-final against Denis Shapovalov after experiencing discomfort in the warm-up.

He underwent an MRI in Mallorca and declared he was planning to play the ATP Finals on Tuesday.

Nadal still indicated his determination to play in a further update provided on Friday, but explained he was taking things slowly.

He wrote on Twitter: "Update? Ok, I'm in London happy to be here. Taking it day by day after Paris (small abdominal tear).

"I need to see how things go, started serving [on Thursday] very slowly. Right now the main goal is to be ready on Monday following the protocols that the doctor gave me."

Nadal, 33, is a 19-time grand slam champion but has never won the ATP Finals, finishing as runner-up twice. He had to withdraw from last year's event due to another abdominal problem he picked up in Paris.

The Spaniard has been drawn in Group Andre Agassi this year along with Daniil Medvedev, Stefanos Tsitsipas and 2018 champion Alexander Zverev, who is scheduled to be his first opponent on Monday.

Novak Djokovic will begin the singles tournament with a Group Bjorn Borg match against Matteo Berrettini on Sunday. Roger Federer and Dominic Thiem are the other participants.

Frances Tiafoe and Miomir Kecmanovic sealed their places in the last four of the Next Gen ATP Finals with straight-sets victories on Thursday.

Tiafoe joined Jannik Sinner in progressing from Group B in Milan with a 4-2 4-2 4-2 defeat of Mikael Ymer.

Second seed Tiafoe missed out on a place in the semi-finals of the season-ending event last year when he was beaten by Jaume Munar, but was not to be denied on this occasion.

The American will face top seed Alex de Minaur, who made it three wins out of three in Group A with an emphatic 4-1 4-0 4-2 beating of Casper Ruud.

Tiafoe went on a five-game streak from 2-1 down in the first set against the Swede at Allianz Cloud and never looked back.

The 21-year-old said: "I'm pretty stoked. I needed this. The second half of this season has been rough for me, so every match is an opportunity for me.

"I love this event and I want to stay. The more you win, the longer you can stay, so I'm happy to be in the semis."

Kecmanovic advanced by seeing off Alejandro Davidovich Fokina 4-1 4-1 4-3 (8-6) in just under an hour.

The Serb will take on Sinner, who went down 4-3 (7-5) 3-4 (3-7) 4-2 4-2 to Ugo Humbert in the final round-robin match.

World number two Karolina Pliskova has confirmed she has split from her coach Conchita Martinez.

Former Wimbledon champion Martinez, who was second in the world herself during her playing career, started working with Pliskova on a full-time basis 10 months ago.

The Czech won four WTA titles in 2019 - the joint-most on the tour - but failed to make it beyond the fourth round of a grand slam after reaching the Australian Open semi-finals in January.

Last week, Pliskova was beaten in the last four of the season-ending WTA Finals by Ashleigh Barty.

"I decided I will no longer work with Conchita," Pliskova wrote on her social media accounts.

"It was tough decision, as the season was great. Thank you for all this year, and wish you only the best. Life is a change!"

Martinez, who coached Garbine Muguruza when she won Wimbledon in 2017, also posted a message on social media.

"Just a little note to inform you that Karolina and I have decided to take different paths next year," she said.

"It has been an incredible year full of great moments and emotions. I wish Kaja and her team the best for the future."

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