Seventh seed Dementieva slumped to Jelena Jankovic of Serbia and Montenegro 6-1, 6-4 while 10th seed Petrova lost to Hungarian Aniko Kapros 6-3, 6-3.
Vera Zvonareva and Svetlana Kuznetsova fared better in a mixed start for the Russians.
Eleventh seed Zvonareva pounded Czech Zuzana Ondraskova 6-3, 6-1 and 30th seed Kuznetsova breezed past American Shenay Perry 6-3, 6-3.
While Dementieva was left disappointed by the defeat, it marked Jankovic's
"It's my first win over a top 10 player...it is my biggest win," the 18-year-old told reporters, "I am just so happy."
Thai Tamarine Tanasugarn, a favourite at the tournament which bills itself as the Grand Slam of Asia, disappointed her fans, losing to Laura Granville of the United States 6-1, 6-3.
But Thai men's 13th seed Paradorn Srichaphan eased past Argentine Jose Acasuso 7-5, 6-4, 6-0.
American Robby Ginepri handily beat Peru's Luis Horna 7-5, 6-3, 6-3 and Chris Guccione won the all-Australian battle over Alun Jones 6-3, 7-6, 6-2.
