Injury problems threaten to overshadow the season-ending Tennis Masters Cup, already rocked by the withdrawal of world number one Rafael Nadal.

Roger Federer, top seed in Nadal's absence, is battling a back injury and Argentina's Juan Martin del Potro has a painful right toe. Nadal pulled out last week citing tendinitis and fatigue.

Any further disruptions would dismay Chinese organisers, who are hosting the event for the last time and voiced anger when Nadal, Andre Agassi, Andy Roddick, Lleyton Hewitt and Marat Safin all withdrew in 2005.

The equivalent women's tournament in Doha has been hit by the double exits of Serena Williams and Ana Ivanovic due to a stomach injury and virus, renewing questions about the rigours of modern tennis.

Federer forfeited his Paris Masters quarter-final against James Blake last week citing a stiff back, but arrived here safely to begin his preparations.

"With overnight painkillers you sort of get over it but I just can't play the way it is right now. Hopefully it will be fine for Shanghai," he said in Paris.

Del Potro has been bothered by a split big toenail since the US Open and was forced to pull out of last month's Vienna Open, beating only one top-10 player since.

The Argentine opens the tournament on Sunday against world number three Novak Djokovic, with France's Jo-Wilfried Tsonga facing Nikolay Davydenko in the later match.

On Monday, Federer plays Britain's Andy Murray and American Andy Roddick starts his campaign against Gilles Simon of France.

Federer, a huge hit with Chinese fans, is chasing the first year-end title hat-trick since Ivan Lendl in 1985-1987 and his fifth Masters Cup overall at the end of a difficult year.

He has a superb record at the Masters Cup, reaching the last five finals and losing only one, when he was struggling with an ankle injury against David Nalbandian in 2005.

But the ex-number one faces a tough opener against Murray, who avenged his US Open final defeat to the Swiss in Madrid on his way to back-to-back Masters series titles.

"Since Wimbledon I've played the best tennis of my life, and hopefully I can keep it going," said Murray, 21.

The Scot heads a list of four new faces, along with Tsonga, del Potro and Simon, at the least predictable season finale of recent years.

Tsonga, 23, had three months out injured this year but muscled into the draw with a surprise victory last week in Paris.

"It's an amazing turnaround," he said.

Serbia's Djokovic is another danger man and he will be hoping to bookend a season which started with his maiden Grand Slam win at the Australian Open.

Simon was a late inclusion thanks to Nadal's withdrawal and Russia's Davydenko is making his fourth straight appearance after another consistent season. Roddick is playing for a fifth time and has reached three semi-finals.

The 'fifth Grand Slam,' featuring the top eight players, is making its fourth and final appearance at the futuristic Qi Zhong Stadium before moving to London as the ATP World Tour Final.

Red Group: Roger Federer (SUI), Andy Murray (GBR), Andy Roddick (USA), Gilles Simon (FRA)

Gold Group: Novak Djokovic (SRB), Nikolay Davydenko (RUS), Jo-Wilfried Tsonga (FRA), Juan Martin del Potro (ARG)