No. 1 Nadal will be hard to topple: Federer

London: There is hardly anything Roger Federer has not achieved in tennis over the past decade but the Swiss admits seizing back the world number one ranking from Rafael Nadal may stretch even his mercurial powers.

Federer has arrived in London for the ATP World Tour finals a distant second to the phenomenal Spaniard in the rankings and while his desire still burns as strong as ever he is not expecting a return to the top any time soon.

"Sure, it's going to be challenging," Federer told reporters from a hotel lounge overlooking the River Thames and the Houses of Parliament.

"It's going to be very difficult because Rafa is playing well and I have to defend my points in Australia and he has no points to defend here in London," added Federer, immaculate in a navy blue suit and tie.

"That's why things look very good for him over the next few months."

Federer, 29, who began the year by winning the Australian Open but subsequently lost the top ranking to Nadal just one week short of equalling the 265 weeks Pete Sampras spent as number one, has not ruled out the possibility altogether though.

"Right now the number one ranking is not something I have in my mind," said the 16-times grand slam champion.

"My goal is to play well here in London and then prepare well for next season and then hopefully, at some stage, I'll try get it back. If not I'll focus on winning tournaments because I enjoy doing that too."

Federer has won the year-ending showpiece four times and, outside the four grand slams, rates it as a priority each year.

"Right now it's about saving energy, getting ready to play the matches," he said. "I've played four of the last five weeks and my game is right where I want it to be.

"It's the ninth time for me and I know the drill. I feel like I've got one more tournament left in me. My body is not screaming for a vacation quite yet," said Federer.

"This is a huge tournament for me. It is a goal of the season to make it here. I hope I can save my best until last."

Federer is in a group with his French Open conqueror Robin Soderling, Briton Andy Murray and Spaniard David Ferrer, who he plays first on Sunday.