WIMBLEDON, England — Defending champion Rafael Nadal withdrew from Wimbledon today because of tendinitis in his knees.

He is the first reigning Wimbledon men's champion to not defend the title since Goran Ivanisevic in 2002 — and only the second in the last 35 years.

The grass-court Grand Slam tournament begins Monday.

"I'm just not 100 percent," the No. 1-ranked Nadal said in a news conference at the All England Club. "I'm better than I was a couple of weeks ago but I just don't feel ready."

His announcement came about 2½ hours after he lost to Stanislas Wawrinka in an exhibition match on grass at the Hurlingham Club in south London. Nadal also lost an exhibition there to 2002 Wimbledon champion Lleyton Hewitt on Thursday.

Nadal has complained about his knees since his fourth-round loss at the French Open on May 31 ended his streak of four consecutive championships at Roland Garros. Later that week, he pulled out of the Wimbledon tuneup tournament at Queen's Club, then went to Barcelona to have tests on his knees.

"I think I reached the limit right now. I need to reset to come back stronger," Nadal said.

Asked what sort of threat the knee problems might present to his career moving forward, he said: "It's not chronic. I can recover for sure."