Nadal, who sat out after the U.S. Open because of tendinitis in both knees, had five winners and 29 unforced errors against Nalbandian.

"It's hard to play worse, which makes it a lot easier to go forward from this," Nadal said. "I didn't go in with a clear enough idea of the match I wanted to play."

The 33rd-ranked Argentine will play Novak Djokovic, who defeated Mario Ancic 7-6 (2), 6-2, in his first semifinal of the year.

It was Nadal's most lopsided loss since Julien Benneteau beat him 6-3, 6-0 in Lyon in October 2004.

"I took advantage of all of Rafa's bad shots," Nalbandian said. "I think Rafa never felt comfortable out there."

The second-ranked Nadal, coming off a 7-6 (5), 6-4 win yesterday against Andy Murray, was slow to Nalbandian's groundstrokes and broken by the third game.

A double-fault in the fifth game set up another break chance for Nalbandian, whose forehand clipped the net to skip beyond Nadal for 4-1. After holding to love, Nalbandian reached Nadal's overhead smash for a crosscourt winner and the set.

The Spaniard's layoff showed as he looped returns out of play or into the net. But Nadal refused to blame fatigue for the loss.

"I don't want to come up with an excuse," he said. "When you lose, you lose. I think the result reflects enough. It wasn't my day. I never felt good at any point."

Djokovic, who reached the semifinals for the 11th time this year, needed treatment on his left ankle before the second set.

Later today, defending champion Roger Federer faces Feliciano Lopez, who is 0-for-5 against the top-ranked player.

Djokovic, the U.S. Open finalist who won Masters titles in Miami and Montreal this year, saved three break points in the opening game despite making six unforced errors.

In the 12th game, Ancic reached a volley near the baseline and sent a crosscourt passing shot beyond Djokovic to force the tiebreaker.

The 29th-ranked Croat, who missed more than six months of the season because of mononucleosis, couldn't return two straight serves from Djokovic, who went ahead 5-1 in the tiebreaker.