MIAMI, FL, USA - In by far the biggest match of her career on Thursday night Johanna Larsson was tough as nails, upsetting No.6-ranked Li Na in the Chinese trailblazer's opening match at the Sony Ericsson Open.

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Larsson, who had never even played a Top 25 player, won a tight first set, 7-5, then went ahead 4-0 in the second, seemingly on her way to a far easier victory. Li's power game caught fire at that point and the match went to a third, where the Chinese had leads all over the place, going up 4-0, holding three match points at 5-4, 4-0 in the tie-break; Larsson overcame it all, 75 67(5) 76(5).

"It's definitely the biggest win of my career," Larsson said. "I got really nervous and messed it up a little bit, but I kept fighting and running, and somehow I was the winner in the end. The match is never over until the last point and I just tried to stay positive. I knew if I kept my level up there was always a chance.

"I never stopped believing."

Li has now lost five matches in a row, beginning with the Australian Open final (Kim Clijsters) then opening matches at Dubai (Yanina Wickmayer), Doha (Klara Zakopalova), Indian Wells (Peng Shuai) and now here. Four of those five losses have been three-setters and this was the second one where she had match point (she held quadruple match point against Wickmayer in Dubai).

Maria Sharapova, who had missed this tournament the last three years due to injury, had no trouble in the feature night match. The No.16-seeded Russian cruised past Croatian wildcard Petra Martic in just 66 minutes, 63 62.

"It has been way too long," Sharapova said of the last time she played here. "This is one of my favorite events. It's crazy I've missed it this many years. I'm just so happy to be back. I love playing in front of this crowd, they're so enthusiastic. They love their sports and have that Latin flavor. It's fun."

Sharapova is a two-time finalist in Miami, finishing runner-up in 2005 (to Clijsters) and 2006 (to Svetlana Kuznetsova). Next for Sharapova is another wildcard, Germany's Sabine Lisicki, who advanced with a 46 63 31 retirement victory over No.18 seed Nadia Petrova, who had to stop due to vertigo.

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Other winners were Peng Shuai, who cruised past No.20 seed Aravane Rezai, 60 64, and Elena Vesnina, who regrouped after missing four match points at 62 54 and beat No.23 seed Yanina Wickmayer, 62 57 64.