Roger Federer admits he is unsure if his back will hold up when the Tennis Masters Cup gets under way in Shanghai on Sunday.

The Swiss star has been struggling with the problem since withdrawing from the Paris Masters last week.

Federer is the top seed at the prestigious eight-player season finale in the absence of the injured Rafael Nadal, and comes in to the event as the two-time reigning champion.

The 27-year-old, who starts the defence of his title against Frenchman Gilles Simon on Monday, has been able to practice pain-free, but admits how the injury holds up in a match is another matter.

"It's way better than last Friday," Federer told reporters. "I've been able to practice without pain.

"(But) I'm not sure (what to expect against Simon) because I haven't been pushing it to the very limit. It just needs as much time as possible."

Also a back-to-back winner of this event in 2003 and 2004, Federer is aiming to join Pete Sampras and Ivan Lendl as the only players with five titles.

But even though he has won four of the last five titles, and finished runner-up in 2005, Federer still has something to prove this time.

The 13-time grand slam champion has won 65 matches and four titles this season, including his fifth straight US Open crown, but his 13 loses have been the bigger story.

It is the first time since 2003 that he has endured a double-digit loss total, and his four titles are the fewest since 2002, when he won three.

While a bout with glandular fever at the start of the year played its part in Federer's struggles, the form of Nadal has also made things difficult.

The Spaniard has won all four of his meetings with Federer this year, including wins in the French Open and Wimbledon finals.

An eight-time champion this year, Nadal also took the world number one ranking from Federer in August, ending a record 237-week run.

Those setbacks appear to have made Federer more determined than ever, though.

"It is a challenge to get back to number one," he said. "I'm here to win. I'm not here just to participate."

Already guaranteed to finish the year as the world number one, Nadal will not be able to trip up Federer again here as the 22-year-old has withdrawn with knee tendinitis.

That's not to say Federer won't have his hands full as he attempts to emerge from the Red Group and improve on his 26 wins in the Masters Cup.

Awaiting him in round-robin play are American number one Andy Roddick, US Open finalist Andy Murray and Simon - a trio of players who have defeated Federer at least once in 2008.

The Gold Group is headlined by Australian Open champion Novak Djokovic, who will be joined by Nikolay Davydenko, Jo-Wilfried Tsonga and Juan Martin del Potro.

The 20-year-old del Potro has been one of the finds of the second half of 2008.

The Argentinian had never played in an ATP singles final prior to winning four consecutive titles during July and August.

Del Potro has soared from 81st to a career-high ranking of eight since April.