(CNN) -- Nikolay Davydenko stunned world No. 1 Roger Federer with a 6-2 4-6 7-5 victory in the semifinals of the ATP World Tour Finals in London on Saturday.
The Russian was back on court just 15 hours after the epic win against Robin Soderling on Friday night which put him into the last four, and he managed to beat the Swiss superstar for the first time in 13 encounters.
The 28-year-old was beaten in the final last year by Novak Djokovic, the player who he knocked out of contention this time by beating the in-form Swede in his final Group B match.
He will have the chance to go one better on Sunday when he plays U.S. Open champion Juan Martin Del Potro, who later beat Soderling 6-7 (1-7) 6-3 7-6 (7-3) in another classic match on Saturday night.
Del Potro gave Federer his only other defeat of the season-ending tournament this week, while French Open finalist Soderling qualified by winning his first two matches.
"I was tired, really tired," Davydenko said. "Between yesterday and today I was feeling like I was moving a little bit slow.
"We'll see tomorrow. Now I have a little bit more time. I want to recover 100%. I hope tomorrow I will be fit for the final."
Sixth seed Davydenko had never before won the first set against Federer, and had lost 22 of the last 23 sets against him.
But this time he overcame a poor start after losing the first six points as Federer's usually reliable serve then fell away, and Davydenko broke him again to go 4-1 up.
Federer broke back but then lost his serve immediately and Davydenko then served out the set.
Federer, who was earlier in the week named ATP Tour champion for the fifth time, fought back in the second set, which went with serve until Davydenko hit a forehand wide to allow the decisive break in the 10th game.
The final set also went with serve until the 11th game as both players dished up some superb tennis for the indoor crowd, most of whom were backing the favorite Federer.
But Davydenko shocked with with a vital break having fought back to deuce, then murdering Federer's second serve with a forehand down the line.
Serving for a breakthrough victory, Davydenko clinched it when Federer sent a forehand into the net on his first match-point.
"I think all my family, everybody who supports me, have waited for this moment because I beat everyone in top 10 except Federer," he told reporters.
"It's a really good feeling. I don't think now I'm number one because I beat Federer, but I know now, if I come to a match and play against number one, number two, number three in the world, I have a chance. That's really good confidence for next year."
Federer, who had been seeking to reach the final for the sixth time, lost the opening set for the fourth successive match of the tournament.
"I guess that's what cost me the match in the end. I tried everything to have a good start but I wasn't able to do it again," he said.
Del Potro showed great spirit to fight back and claim a place in the final after being crushed by Soderling in the first-set tiebreak.
The 21-year-old delivered some bruisingly powerful shots to win the second set, making just two unforced errors as he registered the match's first break of serve in the eighth game.
He then battled back from 4-2 down in the deciding set and the two players slugged it out to force another tiebreak, in which Del Potro moved out to a commanding 4-0 lead before ultimately triumphing.