Williams began the year by winning the Australian Open for her first Grand Slam title in two years.

"I just didn't make the shots I was supposed to make," Williams said. "I probably made 50 unforced errors, which is pretty good to get to 7-6 in the third."

The 14th-seeded Schnyder, who was runner-up at this French Open warmup two years ago, also beat Williams in their only previous match on clay. Williams won the other six meetings.

"I think it's her weakest surface," Schnyder said. "I'm extremely happy that I could do it again this year, because she's the champion to beat this year."

Williams said she does not plan to play again before Roland Garros, which begins May 27.

"I think the best thing for me now is practice," said Williams, whose record is 18-3 this season.

Last month, Williams retired from a match in Charleston, S.C., with a groin muscle problem and pulled out of a Fed Cup match with inflammation in her right knee.

"I'm getting there," Williams said of her fitness. "Actually, not winning today is going to work well for me. I'm going to get even more fit."

In the semifinals, Schnyder will face third-seeded Jelena Jankovic, who routed Elena Dementieva 6-2, 6-1.

Second-seeded Svetlana Kuznetsova beat Dinara Safina 6-1, 6-3. She'll play either Daniela Hantuchova or Anabel Medina Garrigues.

Williams struggled with her consistency in the first set, missing a series of backhands and committing costly double faults. Schnyder, a left-hander with a crafty all-court game, simply kept the ball in play until the American missed.

"I don't have the big muscles, the height, and so I need to get something special to my game and that's what I work on," Schnyder said. "I can play all the spins — the slice and kick and the fast serve."

Schnyder took a 2-0 lead in the second set before Williams reeled off six straight games to force a third set.

Williams saved a break point in the first game of the third set and two more at 2-2 in the third — the first with a drop-shot winner and the second by stepping into the court to angle a backhand winner.

In the tiebreaker, Schnyder took a 5-2 lead but Williams tied it at 5-5. Williams missed a forehand into the net on the next point and Schnyder hit an approach-shot winner on her first match point.

Both players were bothered at times by the wind, which occasionally whipped up the red clay.

"It was strange conditions. It was really windy. It was a different court than yesterday," said Williams, who beat 11th-seeded Shahar Peer in straight sets on Thursday.

Schnyder reached the quarterfinals at the German Open last week. Now she's in her first semifinal of the year.