Stefanos Tsitsipas defeated Roger Federer 6-3, 6-4, to reach the final of the 2019 ATP Finals in his tournament debut on Saturday.
The Greek sixth seed beat the Federer at the Australian Open but had failed twice since then.
Federer was different from the player who dominated Novak Djokovic in his final round-robin match. The Swiss was struggling on serve and hitting a total of 26 unforced errors compared with just five against the Serbian.
However, Tsitsipas looked beyond his age with a performance full of much confidence and grit.
“I’m so proud of myself today, great performance, and once again, the people were great,” he said.
“I really enjoyed myself on the court, and sometimes in matches like these, you wonder how you recover from difficulties and breakpoint down.
The 21-year-old conceded a breakpoint in his first game as the chant of Federer rang around London’s O2 Arena. However, he survived the early scare and broke Federer in the next game, taking advantage of two missed overheads from the former world number one.
After that, it was a case of what might have been for Federer, who dropped six points on his serve in the first set and saw a massive six break points come and go.
The Greek was then made to dig deep to finish the set in a noteworthy 13-minute final game where he saved two break points and needed seven set points to end it 6-3.
Tsitsipas slipped to 15-40 down, but Federer again could not take advantage, spraying a forehand out to give his opponent a match point, and he won with a thundering ace.
A year ago, Tsitsipas won the Next Gen ATP Finals. Now, on his tournament debut, he is one match away from winning the season-ending event, featuring the year’s best eight players.
Tsitsipas first broke into the top 100 of the ATP rankings only 25 months ago.
He will face either Dominic Thiem Alexander Zverev or fifth seed Dominic Thiem in Sunday’s final.