Mauresmo has led the WTA rankings for 31 weeks this year

But the Australian Open and Wimbledon champion suffered a recurrence of her shoulder injury in the process.

The French star was forced to pull out of the recent Stuttgart Grand Prix because of the problem.

Mauresmo, who struggled with Stosur's big serve, said: "I hurt it in a new place, but we'll see once I've cooled down what the situation is."

It was only Mauresmo's second win in four appearances in Zurich.

She won in the first round in 1998, but lost her opening matches in 2003 and 2005.

"It's a relief. Some of my worst memories are from here," Mauresmo added.

Home favourite and fifth seed Martina Hingis had an easier passage into the quarter-finals, easing past Nathalie Dechy of France 6-3 6-3.

Slovakia's Daniela Hantuchova continued her good run in Switzerland with a 7-6 (9-7) 6-2 win over Ai Sugiyama of Japan.

Tatiana Golovin of France, who reached the Stuttgart final, had to retire with an injury while leading Russia's Maria Kirilenko 4-2 in the first set.