ATP World Tour Finals
Venue: O2 Arena, London Date: 22-29 November
Tournament coverage: Live on BBC Two, 29 November, 1400 GMT; live on BBC Radio 5 live 1600 GMT; 5 live web commentary 1400 GMT; live text commentary on BBC Sport website; live coverage on Sky Sports
World number one Federer is a four-time winner of the event
By Piers Newbery
BBC Sport at the O2 Arena
Nikolay Davydenko caused a major upset by defeating Roger Federer for the first time in 13 attempts to reach the final of the ATP World Tour Finals.
Sixth seed Davydenko beat the world number one 6-2 4-6 7-5 in a contest that started only 15 hours after the Russian's final group match.
Federer had been looking to match the five victories of Pete Sampras and Ivan Lendl at the season-ending event, but Davydenko secured a return to the final he also made 12 months ago in one hour 55 minutes.
Davydenko will now face Juan Martin del Potro on Sunday after the Argentine overcame Robin Soderling in a dramatic three-setter in the second semi-final.
"All my family, everybody who supports me, waited for this moment, when I can beat Federer," Davydenko said.
"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 the number one, number two, number three in the world, I have a chance. That's really good confidence for next year."
Federer said: "It's disappointing, not to lose against him, just to lose in the semis.
"Coming so far in a tough group, in a tough tournament, I had hoped to get through to the final and maybe win again.
"But I missed the start again and 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.
"In the second set it was more a story of just trying to hang on, get through to the third. As the match wore on, I actually started to feel better and better.
"I was a couple of points away but just couldn't get it done. He did well. I thought he played strong throughout, even though I don't think it was our best match we ever played against each other."
The semi-final was always going to be a daunting task for the 28-year-old Davydenko, who also reached last year's final but went into the match with an 0-12 record against Federer.
And when asked after Friday's win over Soderling whether he had enough time to prepare for facing Federer, the Russian had simply laughed.
Federer enjoyed the added advantage of a day off on Friday and predictably looked the sharper in the opening moments, winning the first seven points and earning two break points in game two.
But the Swiss found the net on both occasions and quickly regretted it as his game began to unravel, a double-fault, an awkward prod over the baseline and a hooked forehand into the tramlines giving Davydenko the first break at 2-1.
I was moving a little bit slow, but I was concentrating on every game, every point 100%
Davydenko has struggled to make much impression on the Federer game in the past because he lacks a killer weapon, such as a big serve or powerful forehand, but after three tough games in London his groundstrokes were well and truly grooved.
As Federer failed to make many winners off his first serve, Davydenko was able to control the battle for territory by moving his man into wider and deeper positions than felt comfortable.
It began to produce errors from the 15-time Grand Slam champion and a double fault did not help as the Swiss was broken to love to fall a double-break down at 4-1, at which point the chants of "Let's go Roger, let's go!" from the Federer supporters took on a hint of desperation.
Their man calmed those fears when a beautiful backhand lob and heavy forehand winner into the corner helped him cut the deficit with his first break, but Davydenko restored his advantage with a fabulous cross-court backhand pass on his way to 5-2.
The Davydenko serve is generally seen by opponents as an area to attack but the Russian closed out the set with little fuss and made solid progress in the second.
It was Federer who was under pressure again with defeat looming at 4-4 but he survived a tense game from deuce, and Davydenko finally cracked in game 10 as a couple of nervous errors handed Federer two set points.
The world number one let out a relieved yelp when a Davydenko forehand sailed long on the second and it appeared that normal service was about to resume.
Davydenko also reached the final last season
The early stages of the final set were a low-key affair as both men dominated on serve but after Federer held comfortably to lead 5-4, a magnificent overhead return off a Davydenko smash finally got the 17,000 spectators engaged.
But the Russian recovered superbly from 0-30 to level and then stunned those same spectators when he broke serve with a backhand down the line to leave him one service game from a landmark victory.
It was as tense a game as the O2 arena has witnessed this week as Federer recovered from 0-30 to earn a break-back point with a brilliant backhand, only for Davydenko to respond with a rasping forehand into the corner.
And the Russian wrapped it up after an hour and 55 minutes on his first match point before taking the applause of a slightly surprised crowd, while an equally bemused Federer headed straight to the locker room.
Davydenko, who admitted that after Federer's spectacular smash in game 10 of the decider he was thinking "Not again, please," said he was starting to feel the effects of a punishing week.
"I was tired, really tired," he said. "Between yesterday and today I was moving a little bit slow, but I was concentrating on every game, every point 100%. Now I have a little bit more time and I hope to be fit for the final."
In the first of the doubles semi-finals, seventh seeds Max Mirnyi and Andy Ram defeated fifth seed Frantisek Cermak and Michal Mertinak 6-4 7-6 (7-4).