London: Stefanos Tsitsipas shocked six-time champion Roger Federer 6-3, 6-4 to reach the final of the ATP Finals in his tournament debut on Saturday.
The Greek sixth seed, 17 years younger than his 38-year-old opponent, beat the Swiss at the Australian Open but had fallen to him twice since then.
Federer was unrecognisable from the player who dominated Novak Djokovic in his final round-robin match, struggling on serve and hitting a total of 26 unforced errors compared with just five against the Serbian.
But Tsitsipas belied his years with a performance full of confidence and grit, saving 11 out of 12 breakpoints during the match.
Coming into the match, the Greek 21-year-old led the tournament in service games won, with 35 out of 37.
Tsitsipas conceded a breakpoint in his first game as cries of "Let's go Roger, let's go" rang around the O2 Arena but he survived the scare and broke Federer in the next game.
Thereafter it was a case of what might have been for Federer, who dropped just six points on his serve in the first set and saw a whopping six break points come and go.
Tsitsipas was forced to dig deep to see out the set, needing seven set points before he finally closed it out 6-3.
Federer was in deep trouble when Tsitsipas broke him to love in the third game of the second set but he finally made a breakpoint count -- his 10th, to level at 2-2.
Tsitsipas, dominating rallies from the back of the court, broke back straight away with a forehand cross-court winner for a 3-2 lead.
At 5-4 down Federer knew he had to break Tsitsipas for only the second time in the match.
The Greek 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 an ace.
Austrian fifth seed Dominic Thiem plays defending champion Alexander Zverev of Germany in the second semi-final later on Saturday.
Thiem dethrones Zverev to set up London final against Tsitsipas
Dominic Thiem beat defending champion Alexander Zverev to join Roger Federer's conqueror Stefanos Tsitsipas in the final of the ATP Finals in London.
The Austrian fifth seed saw off the big-serving German 7-5 6-3 on Saturday after Greece's Tsitsipas, making his debut at the tournament, had earlier beaten Federer 6-3 6-4.
Thiem is playing in his fourth ATP Finals but during his first three trips to the season finale, he won only three matches and never advanced out of the group stage at the O2 Arena.
Zverev landed 75 percent of his first serves but Thiem was tougher in the big moments, breaking in the 12th game of the opening set and in the sixth game of the second set.
The Austrian also saved all four break points on his racquet.
Germany's Zverev, who beat Novak Djokovic in last year's final, went toe to toe with Thiem in a relatively uneventful first set but served a double fault to lose the opener 7-5.
The seventh seed, 22, regrouped and settled back into his serving rhythm at the beginning of the second set but was broken again to trail 4-2.
Thiem, looking unflustered, fended off a couple of break points in the following game and served out to take the match, winning with a forehand down the line.
"This is a big, big dream coming true for me, it is one of the biggest and most prestigious tournaments of the whole year and I'm getting the chance to play the final," said Thiem.
The 26-year-old, who reached the final of this year's French Open, has been considered a clay-court specialist but joked after winning indoors in Vienna last month and now reaching the final in London he was becoming a "big, big fan of faster hard surfaces".
