The record 15-time Grand Slam champion is now closing in on Pete Sampras' all-time mark of six.

"It's one of the greatest performances I think I've achieved," said Federer, who beat Andy Murray at the O2 Arena on Tuesday night to clinch the ranking ahead of Rafael Nadal.

Federer earned the top year-end ranking after winning his opening two matches at the ATP World Tour Finals. Nadal, who ended Federer's five-year run as the end-of-season No. 1 last year, had a mathematical chance of overtaking Federer for the top spot but lost his opening match at the tournament for the top eight players in the world.

Federer bounced back from an off season in 2008 by reaching all four Grand Slams finals yet again in 2009, and winning two of them. The first championship came at the French Open, making the Swiss great only the sixth man to complete a career Grand Slam. The next one was at the All England Club, where he reclaimed the Wimbledon title by beating Andy Roddick 16-14 in the fifth set and also surpassed Sampras with his 15th major title.

"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," Federer said. "It's a wonderful feeling."

Up next for Federer, who also got married and had twin daughters this year, is trying to match Sampras with a sixth year-end No. 1 ranking. He is currently tied with Jimmy Connors with five apiece.

But this year's achievement was also special because Federer became only the second player to reclaim the distinction after losing it. Ivan Lendl, who was No. 1 from 1985-97, did it in 1989.

At the ATP World Tour Finals, Federer has twice rallied after losing the first set, first beating Fernando Verdasco on Sunday and then Murray on Tuesday. He will next face Juan Martin del Potro on Thursday to guarantee himself a spot in the semifinals of the season-ending tournament.

Del Potro, who has one win and one loss in the round-robin stage of the London tournament, beat Federer in five sets in the U.S. Open final.

"It could be a make-or-break match for us to get through. If that's the case, I'll give it all I have and try to beat him this time," Federer said. "Last time we played, it was a fantastic match in New York."