Chanda Rubin, of the United States also won, defeating Spain's Angeles Montolio, 6-1, 6-3, to advance to a meeting with fourth seed Jelena Dokic of Yugoslavia. Dokic has won two of her last three events.
Schett lost tp Rubin in the first round of the Porsche Tennis Grand Prix at Filderstadt, Germany, last week after reaching the quarterfinals at the Kremlin Cup the week before.
The 25-year-old Austrian is making her sixth straight appearance in Zurich, with her best result coming last year when she upset the third and fifth seeds to advance to the semifinals. She fell to eventual winner Lindsay Davenport.
Petrova takes on new world No. 1 Jennifer Capriati of the United States, who lost in the quarterfinals last week in Filderstadt but gained the top spot when Martina Hingis injured her ankle in the semifinals and was forced to retire. Hingis will be sidelined six to eight weeks.