SHANGHAI, Nov 8: Injury problems are threatening to overshadow the season-ending Tennis Masters Cup, which has already been rocked by the withdrawal of world number one Rafael Nadal.

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

Federer said he was still taking painkillers and avoiding strenuous training as he battles to recover in time for his opening match on Monday.

“I haven’t been pushing it to the very limits because I want to give my back the maximum time of recuperation,” he said.

“Then we will see on Monday what’s going to happen. But so far I’ve been serving without pain, I’ve been playing without pain and that’s definitely a very good sign.”

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 and admitted he was worried his back problem would flare up again.

“Maybe if I push too early in practice then I’m back to where I was last Thursday and then things are not looking good at all,” he said. “I think this problem I have with my back needs as much time as possible.

“Hopefully I’m just going to get ready for Monday and then things are going to be fine.”

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.—AFP