Stefanos Tsitsipas kept his nerve in the key moments to beat six-time champion Roger Federer 6-3, 6-4 on Saturday and reach the final of the season-ending ATP Finals.
Tsitsipas saved 11 of 12 break points and took advantage of an error-filled performance from Federer, who continually put his opponent under pressure only to come up short when it mattered.
The Greek player next faces defending champion Alexander Zverev or Dominic Thiem.
Tsitsipas, who is making his first appearance at the event, saved all six break points he faced in the first set. That included two at 5-3, when he needed seven set points before finally winning a marathon game.
He broke again for a 2-1 lead in the second then saved three break points from 0-40 in the next game before Federer finally converted his fourth to level the set.
Tsitsipas, 21, broke again with a forehand winner and saved two more break points from 15-40 down when serving for the match at 5-4.
He didn’t give Federer, 38, any more chances at a comeback, serving out the match with an ace.
The 17-year age gap between the two players was the largest in the history of the tournament.
Tsitsipas is the youngest finalist in the ATP Finals since Juan Martín del Potro (also 21) in 2009, according to ESPN Stats & Information. He is the 13th player to make the final in his debut in the event and would be the seventh debuting champion with a win on Sunday.
For Federer, it was a surprisingly erratic performance after he played nearly flawless tennis to beat Novak Djokovic in straight sets Thursday to reach the semifinals.
He finished that match with five unforced errors — including two double-faults — but had 26 in this match.
In the first set, he lost only six points on his serve. By contrast, he won 18 points on Tsitsipas’ serve but couldn’t force a break.
Tsitsipas was coming off a three-set loss to Rafael Nadal in his last round-robin match Friday but showed no sign of fatigue.
Reaching the final is another milestone in a breakthrough season in which he also beat Federer on the way to the Australian Open semifinals.
Tsitsipas is now 2-2 against Federer after losing to him in the Dubai final and Basel semifinals this year.
The Associated Press contributed to this report.