Roger Federer and Rafael Nadal will stage the last chapter of their battle to finish the year as world number one when the ATP Tour Finals get under way in London on Sunday.

Federer goes into the Tour Finals at the O2 Arena top of the world rankings, but the Wimbledon champion knows second placed Nadal can still snatch first place if the Spaniard wins the prestigious end-of-year event.

The race for pole position is the latest installment of an enthralling rivalry between two of the sport's all-time greats. Although Federer and Nadal remain friendly enough off the court, the Swiss star relishes his position as tennis's pre-eminent force and has no intention of surrendering it over the next week.

"I am aware of it (the race to finish world number one) and I would be lying if I said I was just here to play well," said Federer, who kicks off his Group A campaign against Spain's Fernando Verdasco on Sunday.

"Definitely I am here to win the tournament and to try to stay number one in the world. It is obvious.

"The competition is really stiff so it is not going to be easy for me or Rafa if he wants to get it.

"I think it is fun that it is on the line for the last tournament of the season. I will try my best and see where it takes me."

While Federer is setting his sights firmly on that number one position, Nadal is less concerned about the rankings.

The four-time French Open champion has endured the most traumatic year of his career, with his parents' seperation proving a significant distraction before injury denied him the chance to defend his Wimbledon title.

Now fully fit again Nadal, 23, claims he is just relieved to be able to test himself against the world's best again.

"I feel perfect in all aspects, mentally and physically," he said. "I will play against the best players in the world so I can lose every match but I can win every game as well.

"For me the main thing is to be here and enjoy this experience. The rest, finishing number one doesn't matter. My only goal is to try to play my best tennis here in London.

"If I am competitive then I am more than happy. To be number one I have to win the tournament. I can't lose any matches so for me it is not a real chance."

British number one Andy Murray and Juan Martin del Potro, who beat the Swiss in September's US Open final, are also in Federer's group.

Group B features Nadal, reigning Tour Finals champion Novak Djokovic, Nikolay Davydenko and Sweden's Robin Soderling, who was handed his place when Wimbledon finalist Andy Roddick pulled out through injury earlier this week.

London is staging the Finals for the first time in the event's 40-year history and, while Federer and Nadal can't meet before the semifinals, the sell-out crowds in the English capital will see a host of other intriguing ties, starting with home favourite Murray's clash against US Open champion Del Potro in the event's opening match on Sunday.

Murray, 23, has long been regarded as a potential grand slam champion, but while the British number one is still to win his first major, Del Potro, 21, has already done just that, fighting back from two sets to one down to beat Roger Federer in New York.

Del Potro hardly eased the pressure on Murray when he admitted that he expected the Scot to be a grand slam champion by now.

The Argentinian said: "I am a little bit surprised he hasn't won a grand slam yet. That was maybe his goal for this year but he can do it next year. I didn't expect to win one myself before next year or 2011.

"We will play a good match. I know his strengths and weaknesses and he knows everything about my game.

"He is the local player and 20,000 people will be cheering for him."