Mauresmo wasted two match points, including one when she served for the match at 5-3 in the third set.

"Obviously the lack of matches showed a little bit in that moment," said Mauresmo, who missed nearly two months following an appendectomy in March. "When you're out for a while you just want to get back to your level as quick as you can. Obviously I'm not there yet and I'm disappointed about that."

The Italian Open is an important clay-court warmup for the French Open, which begins May 27.

"The whole match was pretty tight. I just had to keep fighting and do my best," Stosur said. "I haven't played for nearly three hours in a long time, especially on clay."

Mauresmo, a two-time Italian Open champion, hadn't dropped a set in her five previous meetings with Stosur, but she struggled from the start against the Australian.

Leading 5-3 in the third set, Mauresmo wasted her first match point when she missed a forehand passing shot. Stosur broke back and saved another match point in the tiebreaker when Mauresmo hit a backhand into the net.

On Stosur's first match point, Mauresmo hit a risky lob that Stosur met with a difficult backhand overhead that set up an easy winner for Mauresmo. Stosur hit a powerful return of serve on her next match point, and Mauresmo netted a forehand.

"I got it back to 5-5 and then pulled it out in the tiebreaker," said Stosur, who has 21 career doubles titles but none in singles.

Second-seeded Svetlana Kuznetsova and No. 3 Jelena Jankovic also advanced. Kuznetsova beat Mara Santangelo 6-3, 6-3, and Jankovic eliminated 16-year-old Austrian qualifier Tamira Paszek 6-3, 6-4.