Sharapova has pulled out of next week's event in Toronto

The top seed pulled out 20 minutes ahead of her semi-final against Nadia Petrova with a lower left leg strain.

And with the US Open beginning on 27 August, she said: "I'm not going to sit here and say this or that is the cause because I don't know."

Ana Ivanovic beat Jelena Jankovic 4-6 6-3 7-5 in the other semi-final.

Third seed Ivanovic came from match point down to beat her fellow Serb.

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Jankovic took the first set and had a host of break points in the second but Ivanovic came back to level the match.

Jankovic grabbed an early break in the decider but Ivanovic again fought back.

The 19-year-old Ivanovic saved two match points at 4-5 before breaking second seed Jankovic to win in two and a half hours.

"I played some awesome points when it was important," said Ivanovic. "I never give up. I just try to focus on each point."

Sharapova, who had already announced she will not play in the Rogers Cup in Toronto starting on Monday, said the pain around her shin prevented her from running and disrupted her service motion.

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