updated 6:18 p.m. ET May 16, 2008

Serena Williams withdrew from her quarterfinal match at the Italian Open on Friday due to a back problem, and older sister Venus Williams was beaten by defending champion Jelena Jankovic.

Jankovic will next play second-seeded Maria Sharapova, who rallied past 2005 runner-up Patty Schnyder 6-7 (3), 7-5, 6-2 to gain the semifinal with her 300th career win. Sharapova will become No. 1 in the rankings Monday, courtesy of Justine Henin’s retirement.

Serena 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 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.

Sharapova improved to 7-1 in her career against Schnyder, a Swiss lefty.

“This is just practice for the French, and I want to play a lot, but I’ve got to be careful with what I wish for because I’m having a lot of court time,” Sharapova said.

Jankovic beat Venus Williams 5-7, 6-2, 6-3, with the American committing 57 unforced errors to the Serb’s 24.

“I had a lot of great opportunities, and I made a lot of opportunities for myself. But then I made a mistake,” Williams said.

Cornet will meet sixth-seeded Anna Chakvetadze, who held off Bulgarian qualifier Tsvetana Pironkova 6-2, 3-6, 6-1.

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Jankovic captured last year’s title at the Foro Italico without dropping a set, recording the best week of her career. She improved to 4-3 in her career against Venus Williams, who dropped to 12-6 this year.

Serena Williams has had a splendid start to the year, compiling a 23-2 record and winning three titles. Her withdrawal deprived the tournament of a chance at the first all-Williams final since Serena beat Venus for the 2003 Wimbledon title.

Last week’s Rome Masters also was hard hit by withdrawals, with both semifinals ending prematurely due to players quitting. Many players have complained about the overloaded schedule, but Serena Williams said that was not the source of the problems.

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“I definitely don’t think it’s a bad moment of the season. I think, if anything, maybe it’s the air in Italy this year,” she said, without elaborating.

The 18-year-old Cornet — last year’s French Open junior champion — is having a breakout season. She reached the semifinals of two clay-court events in the United States last month, and was runner-up to Flavia Pennetta at a small clay-court tournament in Acapulco, Mexico, in March.

“I’m happy, but it’s not the same satisfaction when you win a real match,” Cornet said. “I was excited to play against Serena.”