World number one Dinara Safina suffered a shock defeat by France's Aravane Rezai at the Rogers Cup in Toronto on Wednesday.
The Russian won the first set 6-3 but her French opponent hit back to take the next two sets 6-2 6-4.
Top seed Safina was severely hampered by her serve, producing 17 double faults to give the world number 39 victory.
It follows Tuesday's shock defeat of third seed Venus Williams by Kateryna Bondarenko.
Fourth seed Elena Dementieva, well placed in the absence of Safina and Williams, had a regulation 6-3 6-2 win over Japan's Ai Sugiyama.
Jelena Jankovic, ranked fifth, beat Patty Schnyder with a hard-fought 7-5 6-4 victory.
