LONDON - Spain's world number one Rafael Nadal struggled here on Thursday as he lost 6-4, 6-3 to Australian Lleyton Hewitt to cast further doubt on whether he will be able to defend his Wimbledon title.

The 23-year-old would be the first Wimbledon champion since Goran Ivanisevic to fail to return the following year to defend the crown, which he won in an epic victory over Roger Federer last year.

Nadal is suffering from tendonitis in both knees which saw him fade to a stunning defeat in the French Open fourth round by eventual finalist Robin Soderling of Sweden.

He is due to play another exhibition match against Federer's Swiss compatriot Stanislas Wawrinka on Friday.

Nadal has said on his website that he would take a final decision on his participation at Wimbledon after that match - he has conceded that he would not do so if he wasn't 100 percent - at a press conference.

"I love to play at the Hurlingham Club. Today's match was a real test, as close to reality as it could get. I will come back and play again and then make my decision about playing at Wimbledon or not," he said.

However, his uncle and coach Toni Nadal admitted that he did not know whether his nephew would be able to line-up for Friday's match.

Against Hewitt, a former world number one and Wimbledon champion, Nadal appeared to lack mobility, was agitated and on several occasions produced unforced errors.