LONDON (AP) - The world's best tennis players will be trying to one-up each other this week at the ATP World Tour Finals.
Novak Djokovic has already clinched the year-end No. 1 ranking, robbing that aspect from the season-ending tournament for the top eight players in the world. But with the Australian Open champion sharing the Grand Slams this year with Rafael Nadal (French Open), Roger Federer (Wimbledon) and Andy Murray (U.S. Open), there is still one more big title to win in 2012.
``Whoever wins this wins something more,'' Federer said Sunday, a day before the tournament opens at the O2 Arena. ``That's where I think you can feel the importance of this event.''
Federer, a six-time champion at the ATP Finals, is in Group B with David Ferrer, Juan Martin del Potro and Janko Tipsarevic. Djokovic and Murray are both in Group A, along with Tomas Berdych and Jo-Wilfried Tsonga.
Murray will open play Monday against Berdych, with Djokovic facing Tsonga in the evening match. On Tuesday, Federer will face Tipsarevic before Ferrer takes on Del Potro.
``The crowd's going to be on his side,'' Berdych said of his opening match against Murray, who also won the Olympic gold medal this year. ``I possibly cannot get anything harder than this.''
The big name missing from the tournament is Nadal, who hasn't played since June and pulled out of the ATP Finals because of a left knee injury.
``Groups are loaded with great players on both sides and I'm sure it's going to be difficult for the three of us,'' Federer said, leaving Nadal out of the equation. ``Obviously we do miss Rafa, because he also belongs in here.''
Djokovic comes into the ATP Finals after losing last week in the second round of the Paris Masters, where he said he felt physically down.
That is likely to change in London as each player is guaranteed to play at least three matches in the round-robin stage.
``It's expected not to be always in your top form, especially at this time of year. The effects of the long season can influence you physically and mentally, also,'' Djokovic said Sunday. ``I'm sure that I will find the strength, mental and physical strength, to perform my best.''
The ATP Finals is being played in London for the fourth straight year, and Federer is looking for his third straight win and seventh overall. He'll certainly be the man to beat this week playing indoors where the warm and calm conditions suit his game better than anyone else's.
``You just try hard and hope it all works out and it falls into place,'' said Federer, who also won two season-ending titles in Houston and two in Shanghai before the tournament moved to London in 2009.
``Overall, I think I manage my schedule pretty well to make sure that actually the back end of the season doesn't feel like the back end. So it feels more like a priority, being fresh mentally and physically.''
Although Federer recently reclaimed the No. 1 ranking - and broke Pete Sampras' record for most weeks as the world's top player - Djokovic has enough points to guarantee himself the top spot once the season comes to an end.
``The real No. 1, we know who that is,'' Federer said, referring to Djokovic. ``I don't think there should be any debate around that. The rankings is something that shows you how you've played over a 365-day period. OK, it might all change again in two months at the Australian Open.''
Federer also spoke about doping tests, saying he has been tested less frequently in recent years.
He said he would welcome more.
``I don't like it when I'm only getting tested, whatever number it is, which I don't think is enough, sufficient during the year. So I think we should up it a little bit,'' Federer said. ``I think it's key and vital the sport stays clean. It's got to.''