Marin Cilic fought back to keep alive his hopes of reaching the semi-finals of the ATP Finals for the first time with victory over John Isner.

The Croat, who has won just two of 11 matches in four appearances at the event, triumphed 6-7 (2-7) 6-3 6-4.

The result puts Novak Djokovic into the semi-finals, leaving Cilic, Isner and Alexander Zverev to fight it out for the group’s other spot on Friday.

It was the first singles match of this year’s event to go to a deciding set.

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Cilic holds his nerve

Cilic has something of a reputation as a bottler – the most recent example coming last week in the quarter-finals of the Paris Masters where he lost to Djokovic despite having won the first set.

And there was a moment here when it looked like he might not hold his nerve.

Having secured rare back-to-back breaks against the tour’s leading server to win the second set and take an early advantage in the third, he then inexplicably dropped serve to give Isner hope.

But he then showed his determination – via some first-class returning and a wonderful drop volley – to secure a third successive break.

With Isner holding on to force him to serve for the match, Cilic showed no sign of wobbling as he brought up three match points and took victory when the American returned a looping second serve wide.

“It was very difficult. The first set was extremely tough. John played a great tie-break and I had to stay focused,” said Cilic, whose only other Finals victory came against Kei Nishikori in 2016.

“I served really well and I am really pleased with the win. I felt that I had that momentum towards the end of the second set. I felt the ball well and got some good points on Isner’s serves.

“He hit a few double-faults on the break points – that helped – but I stayed patient and I managed to break back and play some good tennis towards the end.”

Isner faces Zverev on Friday, while Cilic takes on the already-qualified world number one Djokovic with one other semi-final place at stake.

The other group is back in action on Thursday for their final round-robin matches, with Roger Federer against Kevin Anderson after Dominic Thiem takes on Kei Nishikori. All four in that group have a chance of progressing.

Isner let down by double faults

Isner was playing the day after learning of the death of one of his close friends, Kyle Morgan, and he wore the initials KM on his shoes.

As the leading server on the Tour, it was no surprise that he began the match with a thumping 138mph ace.

He delivered a total of 17 aces against Cilic but was let down by six double faults, including the one that handed the Croat the crucial break in the second set.

The American is making his debut at the tournament after enjoying his best year to date – having reached his maiden Grand Slam semi-final at Wimbledon, won his first Masters 1000 title at Miami and finished the season ranked inside the world’s top 10 for the first time.

World number one Novak Djokovic became the first man to reach the semi-finals of the ATP Finals after a clinical win over Alexander Zverev and a favourable result in the group’s other match.

After a very physical and close first set, the 31-year-old Serb ran away with the second to beat the German 6-4 6-1.

Djokovic, chasing a first ATP Finals title since 2015, reached the last four when Marin Cilic beat John Isner later.

“I don’t think it was breathtaking tennis but a win is a win,” he said.

“I played well midway through the second set and started to swing through the ball.

“I had not served that well but he made a lot of unforced errors which helped me to win.”

Indeed, it was a double-fault from Zverev that handed Djokovic the first set and a total of 33 unforced errors that contributed to the 21-year-old’s downfall.

Djokovic plays Cilic in his final round-robin match on Friday, while Zverev takes on Isner.

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Are you Mr Bendy, Novak?

It took until the ninth game of the first set for either player to fashion a break point, Zverev missing two in that game.

Having survived that pressure, Djokovic left it until the perfect time to break in the following game – taking the set when the German double-faulted.

It was an unfortunate way for Zverev to fall behind, having been impressive with his serves until that point, delivering seven aces and in one game firing down three successive serves at more than 140mph.

Tipped by many as a future Grand Slam-winner, Zverev showed some of his talent with some beautiful passing shots but he was up against a player who can contort his body to reach almost anything.

Djokovic met one of Zverev’s body serves with an unconventional placement of his racquet that looked more about shielding his face – but of course the return went in – and later Zverev needed two attempts at a smash to put the ball away, when against any other player the first one would have done.

“That’s a first – ‘Mr Bendy’,” the Serb laughed in his on-court interview. “I am relying on my flexibility a lot. I was fortunate to be surrounded with people who emphasised the importance of stretching and it has paid off.”

Djokovic is favourite to win a sixth title at the end-of-season tournament after a remarkable year in which he came back from elbow surgery to win Wimbledon and the US Open and return to the top of the rankings for the first time in two years.

The top two from the two four-player groups qualify for Saturday’s semi-finals, with the winners of those matches meeting in Sunday’s final.

Where did it go wrong for Zverev?

The turning point of the match was at 4-4 on Djokovic’s serve, when Zverev went 0-30 up. But a forehand and then backhand error wiped out the advantage and the German bounced his racquet off the ground in frustration.

Djokovic netted a forehand to give Zverev a break point, which he wasted by returning long, and the Serb then gave him a second chance with a double-fault but the youngster missed that break point when an attempted lob drifted long.

In the end, Djokovic held his serve with a lovely drop-shot and never looked back.

Three errors in a row in the next game gave Djokovic his first break points of the match and while Zverev saved one of them with a smash, he gave the set away with his only double fault of the first set.

The opening two games of the second set were close, Djokovic whacking his shoes with his racquet when he failed to break in Zverev’s first service game having taken him to deuce.

But after that Djokovic turned the screw, with the German taking only three points in the final five games as the 14-time Grand Slam champion wrapped up victory in one hour and 16 minutes.

“It is important to understand the situation – Zverev is playing against the best player in the world right now, while he is playing some of his best tennis. It’s great for him to understand where he is right now,” former British number one Tim Henman said on BBC TV.

“If he really wants to kick on in the Grand Slams – something which hasn’t really happened so far – then he has to test himself against the level and that was it in a nutshell. It’s very easy to be impatient because he has won so much [three Masters 1000 titles are among his nine ATP titles] but he is still only 21 and is on a journey. Bigger and better things are to come.”

Analysis

Former player and BBC commentator Andrew Castle:

This was from a guy [Djokovic] who was nowhere. He was a lost soul in the last two years after winning the French Open in 2016. He had won everything. But he is re-ignited now. I think it is fantastic to see him back at his best. It is brilliant for the game. Everyone will be watching this and thinking ‘that is a high standard’.

This tournament is finally starting to light up. We have had a lot of ordinary matches but that wasn’t one of them. Djokovic didn’t seem to think it was but if that’s the case then I’d like to see him playing ‘well’.

Quad singles and doubles wheelchair tennis events will be added to the schedule at Wimbledon in 2019.

Quad events are currently played at the US Open, the Australian Open and the Paralympics but not at Wimbledon.

The move comes after a quad doubles exhibition match was held during the 2018 Championships, featuring four of the world’s top 10 quad players.

Quad events join the men’s and women’s wheelchair singles and doubles events in the schedule for next year.

Players are eligible to compete in the quad division if they have a permanent physical disability that results in significant loss of function in three or more extremities.

British 2014 US Open champion and world number three Andy Lapthorne, who took part in the quad exhibition match at Wimbledon, said: “It’s been a big goal of mine to play competitive wheelchair tennis at Wimbledon and we got a little taste this year with the doubles exhibition, which was amazing.

“Now the All England Club has given us the opportunity to play competitively next year.

“It’s the biggest event that you can play in tennis, so we’re all really pleased that the work that has gone into this becoming a reality has paid off,” he added.

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Wimbledon finalist Agnieszka Radwanska has announced her retirement from tennis at the age of 29.

The former world number two was runner-up to Serena Williams in 2012 and won 20 WTA titles in her 13-year career.

She finished every season between 2008 and 2016 ranked inside the world’s top 15, but is currently ranked 75th and has not won a title since October 2016.

Radwanska said the decision to retire was “one of the most important” of her life and was “not easy”.

“Unfortunately I am no longer able to train and play the way I used to, and recently my body can’t live up to my expectations,” she added.

“Taking into consideration my health and the heavy burdens of professional tennis, I have to concede that I’m not able to push my body to the limits required.”

Guatemala’s Christopher Diaz-Figueroa has been banned from tennis for two years – with a further 12 months suspended – for match-fixing.

The world number 723 was found guilty of “contriving the outcome of a match” at an International Tennis Federation event in Prague in November 2017.

The 28-year-old was also guilty of failing to report the approach.

He also had an extra year’s ban and a $5,000 fine suspended for three years by the Tennis Integrity Unit.