Alexander Zverev of Germany won the last place in the ATP final semifinals, defeating John Isner, a result that ensured that the world's number one Novak Djokovic finished in the top of the group.
Zverev dropped a penalty kick before breaking at the end of the second to win 7-6 (7-5) 6-3 in London.
The third seed, Zverev, will play Roger Federer in the semifinals.
Djokovic, who beat Marin Cilic 7-6 (9-7) 6-2 later on Friday, will play Kevin Anderson from South Africa.
The Grand Slam Championship 14 times is hard to win to win the tournament after having managed to go through the round with three wins in the three matches.
In doubles, Pierre-Hugues Herbert and Nicolas Mahut won the group, defeating Mike Bryan and Jack Sock 6-2 6-2 in 53 minutes.
Bryan and Sock finished in second place and will face Jamie Murray and Bruno Soares in the semifinals.
|The afternoon session – 14:00 GMT – live on BBC Two|
|Roger Federer (Swi) against Alexander Zverev (Ger)|
|Evening Event – 20:00 GMT – Online radio commentary|
|Novak Djokovic (Ser) against Kevin Anderson (SA)|
Djokovic offers another statement of intent
Djokovic was identified as the man who beat O2 after his remarkable return to world first, with dominant performances against Isner and Zverev, who sent additional warning signals to the rest of the field.
While he was made to work harder for Cilic in the first set, the Serbian produced another impeccable performance to keep his note not to give up a set this week.
Even if the transfer to the semifinals was already guaranteed as the winner of the group, Djokovic refused to let go of the Cilic with an almost perfect service.
The Wimbledon and US Open champions only dropped five points to serve in the game and won 100% points when their first service landed.
The Croatian Cilic only managed to win two points of reception in the first set, unable to penetrate his opponent's impenetrable service once again at a set point in disconnection and to pay the price when he blew under pressure with a forehand in net four points later.
Djokovic missed three break points in a long opening game of the second set and then saw his 31-winning win on the third game.
That served only to emphasize Djokovic's concentration, however, he broke into the next game and again for a 5-2 lead before serving to win in an hour and 36 minutes.
"Obviously, I knew before the match we already qualified and knew who was playing, so it was not easy to approach this match with the desire to win 100%," Djokovic said.
"But this is a unique format and we both wanted to win from the first point. It was a close first set that probably decided the match."
Is it time to get to the big stage?
With Federer's "big four", Djokovic, Rafael Nadal and Andy Murray, all aged 30, Zverev has long been identified as the youngster most likely to fill the gaps when retiring.
The 21-year-old has not yet had a significant impact on the Grand Slams, but after his eight-time champion Ivan Lendl has added his top four to O2 for the first time.
Now, with Federer and eventually Djokovic standing, victories over the week will announce his arrival to a wider audience.
After demonstrating the psychic resistance at the edge of Cilic's opening match, Zverev knew that any victory against Isner would guarantee his place in the last four, despite being overtaken by Djokovic in the second.
As in Cilic, he managed to get the big points against his American to win a match that was not too small between the two players.
Isner was served and taken out
Zverev delivered a 143 mph assault to destroy a set point at 6-5 – the only break point of the first set – and continued to win a stake that led the opener into a break-tie and the latter turned out to be pivotal.
Isner knew he had to win to stay in the tournament with a two-set victory, his only hope of reaching the semifinals, but a vice president mistakenly entered the net on the second point set by Zverev.
The US 6ft 10in is surprisingly built around its massive service, but has been overtaken by Zverev, which has a faster serving rate, a higher percentage of serving and more aces.
And after Zverev took the first break at 4-4 in the second, the victory was quickly ended with a blow to love and allowed him to think of Federer.
"I'm incredibly happy to be in the semifinals," Zverev said. "We had such a good run in the last year.
"John was such a tough opponent and the last two matches could have gone anyway for him. The first set was incredibly tight, always being one or two points against him.
"I came with some big blows in tie-break and that helped because I do not want to be down with him serving for the set."