Russia's Tatiana Panova upset eighth seed Kveta Peschke of the Czech Republic on the opening day of the W&S Open in Cincinnati on Monday.
World number 41 Peschke was the only seeded player in action, and went down 6-4 7-6 (8-6) to the world number 120.
"She always has a lot of unforced errors," said Panova. "You never know what she's going to do. I played quickly and think that surprised her."
Shahar Peer and Aiko Nakamura also made it through to round two.