Serena Williams, the No. 2 seed as well as the defending U.S. Open champion, got past Maria Martinez Sanchez 6-3, 7-5.
It was a rematch of a controversial third-round confrontation at this year's French Open, when Williams accused Martinez Sanchez of cheating. Williams felt a ball went off her opponent's arm in that match and that Martinez Sanchez did not acknowledge it.
Venus Williams, seeded third, opened the evening session at the National Tennis Center Friday with a 6-2, 7-5 decision over Magdalena Rybarikova.
Also advancing Friday was 10th-seeded Flavia Pennetta, who breezed past Aleksandra Wozniak 6-1, 6-1. Pennetta, who has two tournament victories this summer, has lost only six games in six sets during this event.
A win by Pennetta in her fourth-round match against Vera Zvonareva would likely set up a quarterfinal meeting with Serena Williams.
Kim Clijsters, who ended a two-year retirement this year, also reached the fourth round with a 6-0, 6-2 decision over Kirsten Flipkens.
In the day's biggest surprise, Francesca Schiavone ousted No. 8 seed Victoria Azarenka 4-6, 6-2, 6-2.