London - Rafael Nadal vows to return to 100-per-cent fitness as he battles the fluid on both knees which Friday forced the world number one from his Wimbledon title defense.

The sad Spaniard became only the fourth Wimbledon champion after Stan Smith, John Newcombe and Goran Ivanisevic in 2002 who has been unable to defend his title at the All England club.

"Not playing Wimbledon is one of the toughest decisions of my career," said the man who seized the crown from Roger Federer a year ago, paving the way for a rise to the top ATP ranking.

"This situation makes it a bit easier - there was no option."

Nadal was shocked with a fourth-round exit at the French Open as his bid for a record five straight titles died three weeks ago at the hands of eventual Paris finalist Robin Soderling.

Bypassing last week's elite grass tune-up at Queen's club in order to receive treatment on his troubled and painful knees in Spain, Nadal arrived in London on Tuesday and immediately put in three hours on the practise court.

He lost in a local exhibition this week to Lleyton Hewitt and a few hours after going down to Swiss Stan Wawrinka, travelled to the All England club to deliver the death blows to any hopes of a title repeat this year.

"Wimbledon is very important for me. I tried everything and worked hard in the last weeks to arrive here in my best condition. Today was the last test and I didn't feel terrible but didn't feel close to my best either."

"My position is not to start like this. I want to win and my feeling is that I'm not ready to win. It's very painful for me but I cannot play this year.

"I don't feel ready to compete 100 percent for two weeks. I've had some problems in the knees for last few months, but you always try every week, another one, another one."

The not-unexpected pullout opens the door for French Open champion Federer to start moving back towards the number-one ranking.

Should the Swiss make history by winning Wimbledon and achieving the all-time best of 15 Grand Slam singles titles, Nadal would almost certainly fall back to number two.

But the entire situation will surely re-ignite complaints of a killer ATP tournament calendar which chews and and spits out some players - including the irrepressible Nadal, who cannot pass up a clay event and is now perhaps paying the price.

Nadal and his doctors are putting no timetable on his return. But there will be time for reflection for the team to see what can be done in the future once the treatable condition is brought under control.

"I don't know how long I'll be out," said the Spaniard. "I arrived at one of the most important moments of the year with my worst feelings.

"I've had physical problems in two of the more important tournaments for me - Roland Garros and Wimbledon.

"It's tough to accept. Maybe I had some mistakes about the calendar (schedule). I'll analyse and learn for the next times if there are some mistakes. I have time now to recover, that's what I'm gonna try."

Nadal called his dramatic Wimbledon title victory of 2008 in fading light over a tearful Federer "probably the best moment of my career."

"All my life Wimbledon was one of the more special tournaments.

"But sportsmen always play with pain and you never know where the limit is. But now, I'm at the limit. I need to re-set and come back stronger.

"This is not a chronic problem, I can recover for sure."(dpa)