Daniela Hantuchova advances
Hantuchova was leading 6-3, 3-0 and looked headed for a straight-set victory when Erakovic retired with a lower back injury.
Sharapova leads Hantuchova 8-1 in their head-to-heads, with the Slovak's only win against the four-time Grand Slam champion coming when they first played 10 years ago.
Cornet closed out the first set on serve after breaking Rybarikova twice.
The Serb struggled on her serve in the second set but Cornet missed chances to break her midway through the set and then twice dropped her own serve. At 6-5, Rybarikova broke Cornet again to level the match. But Cornet then broke twice at the start of the third set to regain control.
Knapp advanced 6-4, 6-1 against Spain's Lara Arruabarrena and Petkovic won 6-2, 6-3 against Bojana Jovanovski of Serbia, breaking her serve five times.