MANACOR, Spain (Reuters) - Rafa Nadal said he needs to make a full recovery from a nagging knee problem before returning to competition and hinted that his participation in next month's Davis Cup tie against the United States was also in doubt.

The Spanish world number three has not played since crashing out of Wimbledon in a shock second-round defeat by lowly ranked Czech Lukas Rosol in June. He missed the Olympic Games and on Wednesday he withdrew from the U.S. Open.

"The important thing is to recover well and come back when my knee is a hundred percent perfect," Nadal told Reuters television in an interview in his hometown of Manacor in Mallorca.

"We'll see if I will be ready for Gijon, for the Davis Cup. My goal, my dream is to be there if the captain has confidence in me, but it always depends on the captain and the knee."

Due to his aggressive style of play, Nadal has suffered a string of physical problems during his career, including a foot injury in 2005 and more recently with his knees.

He pulled out of the 2008 Tennis Masters Cup with tendinitis, and a year later the same complaint led to him missing the chance to defend his title at Wimbledon.

A knee injury forced him to withdraw from the Australian Open in 2010 and in March of this year he had to pull out of a semi-final clash with Andy Murray at the Sony Ericsson Open in Miami due to a knee problem.

"The injury is not the same (as before)," he said.

"It is an injury that start around February before Indian Wells. I played these months with some problems, like in Miami, but normally I had the control of the pain, of the injury.

"But after Roland Garros the injury got worse and it was impossible to continue competing, and of course the important thing today is to recover as fast as possible after not arriving in perfect conditions in Wimbledon.

"I am practicing hard on the recovery with the physio in the gym, to not lose my fitness, I am a few days outside of the tennis court to try and recover faster."

Davis Cup holders Spain beat Austria 4-1 in April, without Nadal, to set up next month's semi-final against the United States.

(Reporting by Carlos Herrera, writing by Mark Elkington in Madrid)