London - Alexander Zverev set up a last-four clash against Roger Federer in the ATP Finals after seeing off John Isner 7-6 (7/5), 6-3 on Friday while Novak Djokovic will take on Kevin Anderson.
Zverev played American giant John Isner at London's O2 Arena knowing a win by any scoreline would take him through to the semi-finals of the season-ending tournament.
The turning point in the round-robin match came at the end of the 12th game, when the 21-year-old German saved a set point on his own serve with an ace, going on to force a tie-break.
The players swapped mini breaks but the giant Isner then faltered again on his own serve to hand the set to Zverev - meaning he no longer had any chance to qualify for the last four.
In the second set a single break of serve in the eighth game proved decisive as Zverev served out the match.
"I am unbelievably happy to be through to the semi-finals," he told Sky Sports.
"John is such a tough opponent. The last two matches for him against (Marin) Cilic and me could have gone either way. The first set was unbelievably tight. I am just happy to be through. I will prepare myself the best for tomorrow."
Djokovic will play Cilic later on Friday but it is now a dead rubber as Cilic cannot mathematically reach the semi-finals.
Third seed Zverev has won three titles this year, including the Madrid Masters, but although he is often talked about as the leader of the next generation, his record in Grand Slams is poor and he will be desperate to make a statement in London.
Zverev is the youngest semi-finalist at the ATP Finals since Juan Martin del Potro in 2009.
Second-seed Federer, chasing his 100th title, beat Anderson in straight sets to top the Lleyton Hewitt Group on Thursday but both players progressed, meaning early exits for Kei Nishikori and Dominic Thiem.
Djokovic goes into his evening match against Australian Open finalist Cilic with the luxury of already knowing he has qualified top of the Guga Kuerten Group.
The Serbian world number one has enjoyed a remarkable renaissance in the second half of the season after elbow surgery and indifferent form earlier in the season.
He has lost just two matches since the start of July, winning Wimbledon and the US Open along the way and emerging once more as the man to beat as he attempts to pull level with Federer on six ATP Finals titles.
Results on the sixth day of the ATP Finals at London's O2 Arena on Friday (x denotes seeding):
Guga Kuerten Group
Alexander Zverev (GER x3) bt John Isner (USA x8) 7-6 (7/5), 6-3
Novak Djokovic (SRB x1) v Marin Cilic (CRO x5)
Lukasz Kubot/Marcelo Melo (POL/BRA x3) bt Oliver Marach/Mate Pavic (AUT/CRO x1) 7-6 (7/4), 6-4
Mike Bryan/Jack Sock (USA/USA x5) v Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x8)