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

The British number one 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."

Tonight's 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 tonight 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."