Pennetta of Italy, No. 13 in the world rankings, struggled with her serve and was broken five times in the second-round match.
She followed fourth-seeded Zheng Ji of China and seventh-seeded Alona Bondarenko of Ukraine out in the second round on a hot day when temperatures reached 99 degrees.
Argentina's Gisela Dulko beat Zheng 6-3, 6-3 and Czech player Petra Kvitova had a 6-2, 6-1 win over Bondarenko.
The other seeded players gone before the end of the second round were Patty Schnyder (2) of Switzerland, Ana Chakvetadze (3) of Russia, Hungary's Agnes Szavay (5), Canadian Aleksandra Wozniak (6) and Tamarine Tanasugarn (8) of Thailand.
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