London, Nov 25 - Roger Federer clinched the year-end world number one title a year after losing it to Rafael Nadal, regaining the honour with a 3-6, 6-3, 6-1 defeat of Andy Murray at the ATP World Tour Finals.

For the second time in as many matches, Switzerland's 15-time Grand Slam winner had to rally Tuesday after losing an opening set.

The comeback victory in two hours improved his record against the fourth-ranked Scot to 4-6 and marked his second success in a row in that series after winning a Cincinnati semi-final last summer.

Federer now cannot be denied the 2009 top ranking, even if he should surprisingly not win another match this week and number two Nadal were to claim the trophy on Sunday.

Murray slumped to eight double-faults and 31 errors to 30 for Federer.

The Swiss at first looked like running away as he broke in the first game of the evening, only to be hauled back to earth with a Murray break-back a game later which eventually led to the Scot claiming the opening set in 42 minutes, breaking Federer to love in the sixth game.

But the top seed struck back in the second as he settled into a comfort zone to level. A break of the number four for 2-1 in the third and another for 4-1 paved the way to a 2-0 group record for Federer with one last round-robin to play Thursday.

That contest will be a high-voltage re-play of the US Open final, which Federer lost to Juan Del Potro two months ago.

Del Potro struggled with nerves in a fragile win over Fernando Verdasco 6-4, 3-6, 7-6 (7-1) in well over two and a half hours.

Del Potro almost threw away a 5-2 lead in the final set against the Spaniard before escaping.

"I have to play better than the US Open if I want to win. But it's a good chance to advance to semis," said Del Potro of his date with Federer. "I don't know what will happen. But the match will be completely different (than in New York).

"I feel a little tired, but I can play more matches, especially against Roger. I need the victory if I want to be in semis, so I will fight like today." (dpa)