Andy Murray summed up his Barclays ATP World Tour Finals defeat by Roger Federer in London in three words: "I served pants".

The British No1 forged ahead early on but could not prevent a fightback from the top seed as he went on to triumph 3-6 6-3 6-1 at a vibrant O2 Arena - a result that ensures Federer will end the year as world number one.

Murray's first-serve percentage was down at 45% for the match and, with the Scot winning only 38% of his second-serve points and serving eight double faults, he admitted there was no way he could win the match.

"It was a great effort to win a set serving like that," he said. "It was probably the most double faults I served in a match maybe this year. And if you miss (that percentage of) first serves against Roger then it's easy (for him) to dictate points and dictate play.

"Whereas in the first set, at least when I was behind in games, I was coming up with big serves, being able to dictate the points. But after that I served pants."

The result means all four players are still in contention to qualify for the semi-finals, although Federer and Murray will book their spots if they win their respective matches on Thursday against Juan Martin del Potro and Fernando Verdasco.

The Argentinian's 6-4 3-6 7-6 (7/1) victory over Verdasco earlier meant Murray knew a win would secure his passage through and he looked set to do just that after a first set of superb quality.

In front of a packed and noisy crowd, the world number four recovered from an early break to take the opener, but he was struggling to keep a supremely confident Federer at bay and, once the Wimbledon champion broke through in the sixth game of the second set, there was no way back.

The world number one was close to his vintage best for much of the match but Murray, who had beaten Federer in six of their nine previous matches, felt it was his poor play that was the key factor.

"I didn't play well," he continued. "It happens sometimes.

"I didn't play well and I made mistakes. I served so poorly that anyone would have looked good against me tonight because you get a chance pretty much every single point."

Federer empathised with Murray having been on the other side of similar matches between the pair in the past.

He said: "I think we've always had up-and-down matches. So I think we both always come out thinking, 'Hmm, I could have done better'. I just think it's our styles of play, that when they collide, you always feel like you can play better.

"I've walked away many times feeling like I could have done so much better. Today it's his turn. So it's unfortunate for him."

For the Swiss star, the main reason to celebrate tonight's result was the confirmation he can no longer be caught by Rafael Nadal at the top of the rankings this year.

Federer has now finished a season on top of the pile for the fifth time in his career and becomes only the second man after Ivan Lendl 20 years ago to regain the year-end top spot.

"Obviously, that was one of my big goals for this season," he said. "And it's one of the greatest performances I think I've achieved.

"Especially after having a rough 2008, coming back this year and being able to dominate and play at the top when the depth in tennis is so, so great at the moment. I think it's a wonderful achievement. It's a wonderful feeling."

Federer faces Del Potro in the last Group A clash on Thursday in a rematch of this year's US Open final, where the Argentinian twice fought back from a set down to stun the five-time defending champion.

The 21-year-old put in a disjointed performance against Verdasco today, dominating large sections of the match but missing the chance to serve it out in the decider.

Del Potro admits he has found it hard adjusting to his new-found fame and he will take to the court on Thursday having won only three matches since that September triumph.

He said of his reunion with Federer: "We're playing a different surface, different moment. We are fighting for two semi (spots). I have to play better than the US Open if I want to win."

Verdasco was left wondering what might have been as he reflected on another close match only two days after forcing Federer to come from a set down to triumph.

"It was a big opportunity," he said. "But it's not enough to play good tennis to beat these guys. You need to play good tennis and also play very good in the very important moments and don't make mistakes."