US Open champion Sam Stosur made a premature exit at the Indian Wells WTA tournament on Monday while second seed Maria Sharapova advanced in straight sets. Sixth-seeded Stosur lost a see-sawing third round encounter with Russian Nadia Petrova 6-1 6-7 7-6 at the Indian Wells Tennis Garden, before Sharapova finally wrapped up a 6-3 6-4 win against Romania\'s Simona Halep on the fifth match point. Australian Stosur was broken by Petrova when serving for the match at 6-5 in the third set and the Russian capitalised by holding her serve and sealing a hard-fought victory 7-5 in the tiebreak. Stosur had been swept aside by her big-serving opponent in the opening set and then saved one match point in the second set tiebreak before clawing her way back to level the match. Petrova, beaten 7-6 6-7 7-5 by Stosur at last year\'s US Open in the tournament\'s longest women\'s match of the tie-break era, squealed with delight after the Australian pushed a forehand wide to end an encounter lasting nearly three hours. \"I am very excited to be back,\" a beaming Petrova said in a courtside interview after beating Stosur for the fifth time in seven meetings. \"This is only my third win of the year and you could see it when I was just about to close the match. I was a bit tentative. \"My serve was working incredibly (well) until the end of the second set,\" added the Russian, who fired down 15 aces in the match. \"At the end, it was just spot on.\" Petrova, who raised both arms skywards to celebrate her fifth successive appearance in the fourth round at Indian Wells, was delighted to gain a measure of revenge for her defeat at last year\'s US Open.    \"After that loss in New York it\'s been hard,\" said the 29-year-old Russian, a winner of 10 WTA Tour titles. \"I wanted to turn everything around. I wanted to win and walk off the court with a \'W\'. \"It was a tough battle in the third set, and I was really pumped when I broke her at 6-5. When she was serving for the match. I was pumped. I knew this was my time to win the match.\" The Russian, once ranked as high as third in the world but has now slipped to 33rd, will next meet compatriot Maria Kirilenko, who beat Spaniard Lourdes Dominguez Lino 6-2 4-6 6-4. Sharapova, the 2006 champion, led Halep 6-3 and 5-0 before inexplicably losing her way. The Romanian held for 1-5 and the Russian then squandered three match points on serve before being broken in the seventh and ninth games. After several protracted baseline rallies punctuated by squeals and grunts by both players, Sharapova finally sealed victory on her fifth match point when Halep netted a backhand. \"I started rushing and playing long in the second set,\" said Sharapova, who will next face Italy\'s Roberta Vinci, a 6-7 6-0 6-4 winner against Slovakian Dominika Cibulkova. \"Things weren\'t going my way and it was really tough but I pulled through.\" In other matches, seventh-seed Marion Bartoli of France pummelled South African Chanelle Scheepers 6-2 6-0 and Czech Lucie Safarova advanced when former French Open champion Francesca Schiavone became another victim of the widespread viral infection at Indian Wells.   The 10th-seeded Schiavone, who won her only grand slam singles title at Roland Garros in 2010, withdrew because of a stomach virus when trailing Safarova 2-6.   Defending champion Caroline Wozniacki of Denmark is scheduled to play Swede Sofia Arvidsson in the third round later on Monday.     (Reporting by Mark Lamport-Stokes; Editing by Peter Rutherford)