Germany’s Alexander Zverev sealed his place in the ATP Finals semi-finals with a straight-set win which knocked his American opponent John Isner out of the season-ending event.
Both men dominated their service games, but Zverev nicked the first-set tie-break before breaking at the end of the second to win 7-6 (7-5) 6-3 in London.
Third seed Zverev will play Roger Federer in the semi-finals on Saturday.
Zverev’s win means Novak Djokovic is through as group winner.
The Serbian world number one, who meets Marin Cilic in his final match on Friday, will play South Africa’s Kevin Anderson in the last four on Saturday.
With the ‘big four’ of Federer, Djokovic, Rafael Nadal and Andy Murray all in their 30s, Zverev has long been identified as the young player most likely to fill the void when they retire.
The 21-year-old is yet to make a meaningful impact in the Grand Slams but, after adding eight-time major champion Ivan Lendl to his team, has reached the last four at the O2 for the first time.
Now, with Federer and possibly Djokovic standing in his way, back-to-back victories over the weekend would announce his arrival to a wider audience.
After showing mental resilience to edge past Cilic in his opening match, Zverev knew any victory against Isner would guarantee his place in the last four, despite being outclassed by Djokovic in his second.
As with the Cilic match, he delivered on the big points against the American to win a match in which there was little between the two players.
Isner out-served and knocked out
Zverev delivered an 143mph ace to wipe out a set point at 6-5 – the only break point of the first set – and went on to earn a hold which took the opener into a tie-break and ultimately proved pivotal.
Isner knew he had to win to stay in the tournament, with a two-set victory his only hope of reaching the semi-finals, but mistimed a forehand into the net on Zverev’s second set point.
The 6ft 10in American’s game is unsurprisingly built around his massive serve but he was outgunned by Zverev, who had a faster serve speed, higher first-serve percentage and more aces.
And, after Zverev took his first break point at 4-4 in the second, victory was quickly wrapped up with a hold to love and allowed him to start thinking about facing Federer.
“I’m unbelievably happy to be in the semi-finals,” Zverev said. “I’ve had such a good run over the last year.
“John was such a tough opponent and the last two matches could have gone either way for him. The first set was unbelievably tight. It’s always one or two points against him.
“I came up with some great shots in the tie-break as well and that helped because don’t want to be down with him serving for the set.”