Roger Federer said ending the year as world number one has been one of the greatest achievements of his career.
The 28-year-old lost the number one ranking to Rafael Nadal towards the end of 2008, but has regained the top slot after a superb season.
"It's a wonderful feeling," said Federer, who guaranteed the number one position with victory over Andy Murray.
"It was one of my big goals for this season. It's one of the greatest performances I've achieved."
The 15-time Grand Slam champion said it had been one of his most fulfilling years, particularly as he has remained injury free after spending the early part of the 2008 season recovering from glandular fever.
The year began with tears, when he was defeated by Nadal in the Australian Open final in February, just six months after losing his Wimbledon crown and number one ranking to the Spaniard.
But his year got better with time: in April he married his long-term girlfriend Mirka Vavrinec; in July, he regained his Wimbledon title; and two weeks later, he became a father for the first time when his new wife gave birth to twins.
Federer, who also won five titles on tour, said his success had been partly due to his ability to raise his game when it matters most.
"Playing so well at the most important moments, it's been the key this year, and staying healthy, especially after having a rough 2008," he said.
"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 received a warm reception from the 17,500 fans at the O2 Arena before his ATP Tour Finals victory over Murray and went on to outplay an opponent he had lost against six times in their previous nine meetings.
"I've walked away (against Murray) many times feeling like I could have done so much better. Today it's his turn and it's unfortunate for him," said Federer.
"We've always had up-and-down matches. We both always come out thinking we could have done better. Maybe we have a playing style that makes the other player not play the best tennis.
"The wheels came off for Andy and it just happens sometimes. Of course, I had my doubts that I was going to come through, having lost so many times in three sets against him.
"If feels good, but no more than that. I think this is more the time to celebrate number one rather than celebrate victory over Andy."
Federer becomes the third player in history - after Jimmy Connors and Pete Sampras (who holds the record with six) - to finish top of the rankings for a fifth time.
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