ROME (AP) — Serena Williams withdrew from her quarterfinal match at the Italian Open on Friday due to a back problem.

Williams' back froze suddenly while she was warming up on a practice court for her match with French qualifier Alize Cornet.

"It just happened all of a sudden in practice," Williams said before heading to a hospital for tests. "I just went for a shot and then my back got a little stuck."

Williams did not appear in much pain, carrying a large pocketbook over her left shoulder as she entered a news conference immediately after her withdrawal.

"I didn't feel anything the last match, and I don't expect this to cause any problems with my preparation for the French," she said. "It just happened, you know, all of a sudden. I feel like I'm going to be good going into Roland Garros."

This tournament is an important clay-court tuneup for the French Open, which begins May 25.

In the semifinals, Cornet — last year's French Open junior champion — will meet sixth-seeded Anna Chakvetadze, who held off Bulgarian qualifier Tsvetana Pironkova 6-2, 3-6, 6-1.

Pironkova notched the biggest win of her career Wednesday when she upset top-seeded Ana Ivanovic, and she appeared out of energy in the third set.

Chakvetadze was trying to figure out what went wrong for her in the second set.

"I don't know what happened with me," she said. "I felt like I was playing against myself.

The match was characterized by long baseline rallies, and both players had their right shoulders checked by a trainer at various points.

"Hopefully the trainer will help me today after the match," said Chakvetadze, who won the seventh title of her career at the Paris Indoors in February.

The 64th-ranked Pironkova was playing her first Tier I quarterfinal.

In the other half of the draw, second-seeded Maria Sharapova faced 2005 runner-up Patty Schnyder, and defending champion Jelena Jankovic was up against 1999 winner Venus Williams.