The eighth-seeded Russian will face second-seeded Marion Bartoli or fourth-seeded Agnieszka Radwanska, who were playing Friday.
Petrova will appear in her first final since August in Los Angeles, and she is seeking her first title since winning the Paris indoor event in February 2007. She has struggled much of this year, losing her opening match in seven of 11 tournaments.
The match was delayed by rain and interrupted at 3-3 in the first set. When play restarted, Petrova broke when she forced an error with a strong service return before another 30-minute rain delay.
In the second set, Petrova fought off two break points in the opening game and again at 3-3 before breaking Stosur at love. She clinched the win with her fourth ace.
Ailing Davenport expects to play Wimbledon
AUSTRIA ORGANISERS CONSIDER CHANGES
Final Hall of Fame Game canceled because of rain
Mauresmo holds off sixth-seeded Alize Cornet
