Top seed Maria Sharapova [Images] pulled out before her Los Angeles Classic semi-final against fellow Russian Nadia Petrova because of a strain in her lower left leg on Saturday.

The fourth-seeded Petrova will face third seed Ana Ivanovic of Serbia in the final, who had earlier fought off two match points to beat compatriot Jelena Jankovic 4-6, 6-3, 7-5.

Sharapova, who won last week's San Diego Classic, needed almost three hours to defeat Michaella Krajicek in the third round and survived a tough two-setter against Elena Dementieva in the quarter-finals.

"When I started to warm-up it got worse and worse," Sharapova told reporters. "I could not run for more than two or three balls."

World number two Sharapova was treated by a doctor and trainer for two hours, but still could not make it on court.

"I had it all. Anti-inflammatories, deep tissue massage, an ice bath, I even had acupuncture in my ear," said Sharapova, who had a similar injury in 2006.

"I'm not going to sit here and say this or that is the cause because I don't know."

Sharapova is not scheduled to play again until she defends her U.S Open title in New York from Aug 27.

Petrova said she was disappointed not to get the opportunity to test herself against Sharapova.

"I was really looking forward to playing a big match because it was about time to play someone who has won a lot of titles and won a lot of grand slams," Petrova said.

Petrova trails Ivanovic 3-2 in career meetings, including a fourth-round defeat to the Serb at Wimbledon this year.

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