RAFAEL Nadal said he must fully recover from a nagging knee problem known as Hoffa's syndrome before returning to competition and was a doubt for next month's Davis Cup semi-final against the United States.

Spain's world number three has not played since suffering a shock second round defeat in Wimbledon by lowly-ranked Czech Lukas Rosol in June. He missed the Olympic Games and on Wednesday 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.

DIFFERENT INJURY

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

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

"But after (winning the French Open at) 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 condition at Wimbledon."

Spanish tennis federation (RFET) doctor Angel Ruiz Cotorro said Nadal was suffering from a problem known as Hoffa's syndrome which is a swelling around the tendon just below the kneecap in his left leg.

"It is uncomfortable and painful, but it is not serious," Cotorro was quoted as saying on the Marca website.

"He is recovering well. Next week they will do tests and if the result is positive we will continue with the same treatment."

Nadal added: "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 a semi with the U.S. on September 14-16. (Reuters)