The Russian breezed to a 6-4, 6-3 victory, denying Williams a chance to make the Paris quarterfinals for the first time since 2006.
With the win, Petrova will gain the Round of 8 for the first time in five years. There she will face fifth-seeded Russian Elena Dementieva, who downed South African Chanelle Scheepers 6-1, 6-3.
Also logging a big win Sunday was Belgian star Justine Henin, who triumphed over Maria Sharapova in a match that was completed after being suspended due to darkness Saturday night.
The match resumed tied at two sets, and the four-time French Open champ proceeded to take the final frame, notching a 6-2, 3-6, 6-3 victory over the 12th-seeded Sharapova, who made her earliest Paris exit since 2003.
Henin will face Australian Samantha Stosur in the quarterfinals.
Third-seeded Dane Caroline Wozniacki also won Sunday, defeating Italian Flavia Pennetta in a tough 7-6 (7-5), 6-7 (4-7), 6-2 victory.
Copyright 2010 United Press International.