Williams has not lost to Sharapova since 2005 and is 5-2 overall against her. This was their first meeting on clay
- Sharapova was coming off her first victory on clay last week at Amelia Island.
"It was an up-and-down match," said Williams, trying for her third tour title this year. "I don't think I played my best. Actually, I played far from my best."
She said she wasn't bothered by an ankle she sprained during a match Thursday.
No. 9 seed Vera Zvonareva also advanced to the semis, upsetting defending champion Jelena Jankovic 6-2, 3-6, 6-2.
Zvonareva lost to Jankovic in the semifinals earlier this year in Miami. The two have split their six career matches.
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"She's one of the world's top players," Zvonareva said. "I just tried everything that I didn't try in Miami, I guess, and it worked better for me today."
Zvonareva, ranked No. 17, meets fourth-seeded Elena Dementieva and Williams faces unseeded Alize Cornet of France in the $1.3 million tournament on Saturday. Dementieva beat No. 7 seed Patty Schnyder 6-4, 6-4 and Cornet eliminated Agnes Szavay of Hungary 7-5, 6-4.
Williams' best finish at this event was runner-up five years ago, the same year Sharapova made the field as a 15-year-old qualifier and lost in the first round.