Russian Nikolay Davydenko stunned Roger Federer 6-2, 4-6, 7-5 to book a place in the final of the ATP World Tour Finals and a date with Argentine Juan Del Potro Sunday.

Davydenko sprang the surprise of his career on top seed Federer, earning his first victory in the series for the Russian in 13 matches after losing his first dozen to the world number one.

Davydenko admitted he came to the court thinking he was in for another losing scenario. He, therefore, played without any pressure.

'I thought about losing and going home and the next day already the Maldives. That was really on my mind. I didn't care, I had no pressure.'

The run of disappointment began in 2002 for the plucky Russian, who leads Del Potro 2-1 with their last meeting in 2008.

'Finally I can beat Federer, because I beat everyone in top 10 except him,' said the delighted winner. 'I was thinking it (was) coming maybe 2010 or 2011. But in 2009, end of the season, it's a good feeling.'

Davydenko will test himself in a year-end final for the second year in succession after losing in 2008 in Shanghai against Novak Djokokvic.

Del Potro managed only 13 aces to the 18 of Swede Robin Soderling in a 6-7 (1-7), 6-3, 7-6 (7-3) fighting comeback in two hours 11 minutes in which the Argentine lost serve only once.

The last Swede to reach the ATP final was Stefan Edberg in 1990. US Open winner Del Potro became the first Argentine to reach the final since David Nalbandian won the title in 2005 over Federer.

The Swiss ends 2009 on 29-7 in the year-end championships after appearing in his seventh semi-final in eight tournament appearances.

The loss was only his second in year-end semis after losing to Lleyton Hewitt in China seven years ago.

Sharp play from Davydenko prevented the Swiss from claiming a fifth season wrap-up trophy and limiting him to four this season, including the French Open and Wimbledon.

'Sure, it's disappointing. But not to lose against him; just to lose the semis,' said Federer. 'Coming so far in a tough group, in a tough tournament, I had hopes 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 at the end,' he said after starting all four of his matches this week by dropping the first set.

Davydenko broke twice to sweep the first set and got his concluding break in the penultimate game of the match with an untouchable down the line winner for 6-5.

The unheralded Russian closed it out on his first match point in the next game to claim victory in just under two hours with 27 winners and 21 unforced errors.

'I picked sometimes wrong choice of shots right from the start, that let him recover, and not be a break down,' said Federer.

'After that, I didn't serve very well in the first set at all. So that gave him obviously opportunities to start to find his range from the baseline.

'I thought he played strong throughout, even though I don't think it was the best match we ever played against each other.'

Federer admitted that slow starts in London doomed his chances.

'I tried everything to have a good start, I wasn't able to do it again, so, sure, it's disappointing.

'The important thing is that your serve works, you know. Maybe I just couldn't get my serve working in the first set this week. On top of that, I played top guys. It's just not easy.'

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