MADRID (AFP) - Rafael Nadal's defence of his Wimbledon singles title is hanging in the balance after the world number one admitted he was struggling to overcome a bout of tendinitis in both knees.

The Spaniard pulled out of this week's warm-up tournament at Queen's and is facing a race against time to regain 100 percent fitness, without which he said he would not try to defend his crown.

"I have been playing with pain in my knees for some months now and I simply can't go on like this," Nadal wrote on his website, www.rafaelnadal.com, on Tuesday.

"The pain was limiting certain movements in my body, which affected me mentally as well.

"After the tests, and with the appropriate treatment, we have decided to travel to London next Tuesday, June 16.

"I am going to give 200 percent to be ready for the most important tournament in the world. The tournament that I always dream about. I will not go out and play, especially on the Wimbledon Centre Court, if I am not 100 percent ready to play.

The player's uncle and lifelong coach Toni Nadal said in Paris last week that the 23-year-old had injured his right knee and was doubtful for Wimbledon, which starts on June 22.

Nadal's four-year reign at Roland Garros came to a shock end on May 31 when he was beaten in the fourth round by Sweden's Robin Soderling.

On Friday, he announced he had withdrawn from this week's Wimbledon warm-up tournament at Queen's after being told by doctors he needs to rest his knees.

Nadal's victory at Queen's last year proved to be the perfect platform for an assault on the Wimbledon crown with the Spaniard overcoming Roger Federer in a classic, five-set final.