Sharapova said the pain from a lower left leg strain came on suddenly on Saturday afternoon, and despite feverish efforts to combat it - with ice, massage and accupunture - she didn't feel able to play.

"When I started to warm up, it was getting worse and worse," said Sharapova, whose exit sent fourth-seeded Russian Nadia Petrova into a final against third-seeded Serbian Ana Ivanovich.

Sharapova said she felt discomfort around her shin, which interfered especially with her serve.

"I couldn't lift my foot up on the serve," she said. "I could not run to more than two or three balls. I was in the treatment room trying to get everything done to it, but it didn't help."

Sharapova won her first title of 2007 when she triumphed at San Diego last Sunday.

She remained optimistic that her success there would prove a springboard to a successful defence of her US Open title, and that her current injury would be resolved soon.

"You never know, but I hope it is only a couple of days," said Sharapova, who said she had a similar ailment in Australia two years ago that didn't linger.

Petrova was disappointed to miss out on a chance to test herself against the world No. 2.

"It's always good to be in a final, but it's good to make your way through," she said. "I'm really disappointed. It's a full stadium. I had a lot of people coming watch me and support me. I was looking forward to that."

Ivanvic's progress to the final couldn't have been more different as she had to save two match points en route to a 4-6, 6-3, 7-5 victory over fellow Serbian Jelena Jankovic.

Ivanovic fired 12 aces in the 2hr 28min match and rallied from a 4-1 deficit in the third set.

With the victory, Ivanovic insured she will reach a career high number four in the world rankings on Monday.

That will still leave her one behind Jankovic, who climbed from from 12th at the start of 2007 to No. 3 in the world thanks to four tournament titles this year.

But Jankovic, the second seed who was runner-up here last year to Russian Elena Dementieva, ran out of steam in the third set against her big-hitting compatriot.

Ivanovic broke Jankovic in the seventh game of the third set to narrow the gap to 3-4. However, she fell behind 15-40 on her serve in the 10th game, saving one match point with a delicate drop volley and the next with a forehand winner on the heels of a booming serve.

She broke Jankovic in the next game - rallying after Jankovic took a 40-0 lead with five consecutive forehand winners.

She took the match in the 12th game on her second match point.

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