Petrova, who was eliminated in the fourth round of the U.S. Open by Melanie Oudin, claimed the championship of this event last year with a victory over Bethanie Mattek-Sands.
Fifth-seeded Melinda Czink earned a quarterfinal match-up with Petrova by downing Severine Bremond Beltrame 6-4, 6-3.
No. 8 seed Julia Goerges advanced with a 6-2, 6-3 decision over Rebecca Marino and Alla Kudryavtseva surprised No. 6 Jill Craybas 7-5, 6-2.
Third-seeded Aleksandra Wozniak will play her second-round match Thursday against Regina Kulikova. The winner will meet Kudryavtseva in the quarterfinals.
Copyright 2009 by United Press International.