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.
"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.