|Venue: O2 Arena, London Dates: 11-18 November|
|Coverage: Follow live coverage across BBC TV, radio, the BBC Sport website & mobile app. Live text commentary available on selected matches.|
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.
Zverev himself later later seemed to imply that fatigue had perhaps also played a part, saying the length of the tennis season was "ridiculous".
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.
|Group Gustavo Kuerten|
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 against an apparently tiring Zverev in one hour and 16 minutes.
"The issue is that our season is way too long. But I've said it before," Zverev, whose 55 match wins this year is more than any other player, told reporters.
"We play for 11 months a year. That's ridiculous. No other professional sport does that."
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'.