The unheralded American downed the second-seeded French woman 5-7, 6-4, 6-3 in the second-round clash.

Osterloh, ranked 106th in the world, received a wildcard invitation into the draw. She had beaten fellow American Jill Craybas in the first round while Bartoli, as one of the top four seeds, enjoyed a bye and was playing for the first time since she fell to Venus Williams in the Wimbledon final.

The defeat, in 2hr 23min, meant that world No. 11 Bartoli failed to build on her success at the All-England Club, and saw Osterloh even her head-to-head record with the French player at 1-1.

Osterloh booked a quarterfinal meeting with eighth-seeded Austrian Sybille Bammer, who ousted American Meilen Tu 6-4, 6-2.

Bammer, 27, broke Tu three times en route to the victory.

Third-seedded Daniela Hantuchova and No. 7 Katarina Srebotnik both advanced to the quarterfinals.

The remaining quarterfinal berths will be on offer on Thursday.

Top-seeded Anna Chakvetadze of Russia, seeking to build on last last week's singles' title in Cincinnati, will face Eleni Danilidou of Greece.

Fourth-seeded Patty Schnyder of Switzerland will play Yaroslava Shvedova of Russia.

Schnyder reached the finals here last year but lost to Kim Clijsters of Belgium, who retired from the tour in May.

Tatiana Golovin of France, the sixth seed and a semifinalist here last year, meets Sania Mirza of India on Thursday, while No. 5 Shahar Peer of Israel will face Olga Govortsova of Belarus.