The three-time Warsaw runner-up rallied to beat qualifier Kateryna Bondarenko of Ukraine, 2-6, 6-2, 6-1.
Williams Wednesday topped fellow non-seed Michaella Krajicek of Netherlands, 6-4, 6-3.
Russia's Anna Chakvetadze beat Tathiana Garbin of Italy, 6-2, 6-2; Alona Bondarenko of Ukraine came back to beat Poland's Agnieszka Radwanska, 0-6, 6-4, 6-3; and fifth-seeded Russian Nadia Petrova was ousted by unseeded Italian Mara Santangelo, 6-3, 6-3.
The tournament's eventual winner will earn $88,265.
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