Defending champion Rafael Nadal advanced to the French Open semifinals and extended his record clay-court winning streak when Novak Djokovic retired with a back injury after losing the first two sets.

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Nadal led 6-4, 6-4 when Djokovic quit three points into the third set.

Djokovic, 19, needed treatment for a back problem during the match and quit at 15-30 in the first game of the third set.

"It's not nice to win like that," Nadal said. "I'm sorry for him. But this is good for me to head to the semifinals."

Nadal plays Croatian Ivan Ljubicic in the last four and remains on course for a much-anticipated showdown with world No. 1 Roger Federer in Sunday's final.

The fourth seeded Ljubicic easily dispatched home favorite and unseeded Julien Benneteau 6-2, 6-2, 6-3.

"At the beginning of the second set I had the feeling he was injured," said Nadal, who has now gone 58 consecutive matches unbeaten on clay. He is 12-0 at Roland Garros.

"I think it is a very tough tournament with a lot of long matches, but I'm not tired. I feel good and now I have to concentrate on the semifinal."

Nadal is only a year older than Djokovic but physically it was man against boy as the heavily muscled Mallorcan asserted his authority on a sunny center court.

Djokovic had never been beyond the third round of a grand slam before and ousted three seeds on his way to the last eight but he lost his opening two service games against Nadal and never recovered.

The Spaniard also lost his first service game before controlling the opening set and he took it with a skewered forehand into the corner of the court.

He was soon 4-1 up in the second with Djokovic needing a medical timeout to treat his back and his left thigh.

The Serbian bravely broke back for 2-4 but an errant backhand cost him the set.

Nadal characteristically clenched his fist and Djokovic, faced with overturning a two-set deficit against a seemingly invincible opponent, called it quits soon afterwards.

"I was only serving at 40 or 50 percent maximum," said Djokovic. "I don't think it would have been good for me to continue playing with the pain in my back. This is not the only tournament in the year."

He added optimistically: "I think I had control of the match. I just didn't finish it in the right way when I needed to."

For Djokovic, the French Open ended with a retirement for the second successive year. He quit in the second round last year with breathing problems and cramping.