Roger Federer insists reports of Rafael Nadal's demise are premature - but has warned the Spaniard he will not give up his number one ranking again without a fight as the pair prepare to do battle at the Barclays ATP Tour World Finals.

In a complete reversal of fortune from last season, when Nadal took over as the best player in the world and all the talk was of Federer's decline, the Swiss has enjoyed one of the best campaigns of his career while his rival has been dogged by injuries.

As Federer was completing his set of grand slam titles at the French Open and then winning a world record 15th major crown at Wimbledon, Nadal was battling ongoing knee problems.

Doubts have often been expressed about the 23-year-old's longevity, given his history of tendinitis and his physical style of play, and he has not won a title since returning to the Tour in August.

Nadal has consistently reached at least the last four of tournaments, though, at a stage of the season when he does not traditionally do well, and can pip Federer to top spot in London at this week's prestigious end-of-season event.

The world number one said of his great rival: "I don't think we should exaggerate. The guy has had the most incredible first five months of the year.

"Okay he missed Wimbledon and couldn't defend (his title) but other than that he's seemed okay. He won the Australian Open, he played some incredible matches and he's still got a chance to finish number one in the world. To me that doesn't seem like a horrible scenario to have."

Realistically, Nadal needs to win the title to usurp Federer, and their respective records make that outcome unlikely, with the Spaniard never having reached the final before while Federer won four titles from five successive finals between 2003 and 2007.

In addition, the 28-year-old can become only the second man since Ivan Lendl 20 years ago to regain the year-end number one ranking, and he said: "I'm aware of it. I'd be lying if I said I'm just here to play well.

"I'm here to win the tournament and try to stay number one in the world. Competition's really stiff so it's not going to be an easy task for me or Rafa if he wants to get it but it's fun that it's on the line for the last tournament of the season."

Nadal, though, insists returning to the top of the pile is not his main priority.

"If I win the tournament it doesn't matter if I'm number 10," he said. "The ranking is important, yes, but not the most important thing."

Federer faces Fernando Verdasco in Group A on Sunday evening after home favourite Andy Murray has taken on US Open champion Juan Martin del Potro, while Nadal meets Robin Soderling on Monday and Nikolay Davydenko faces defending champion Novak Djokovic in Group B.

The tournament has come to London's O2 Arena after four years in Shanghai, with 250,000 spectators set to make it the biggest indoor event in history.

Federer added: "Coming here and seeing how much excitement there is around this event and knowing how crazy this part of the world is for tennis, we're very excited. And having a huge stadium, I'm sure it's going to be a great end to the season."