Top seed Anastasia Pavlyuchenkova was taken the distance before claiming a 6-2 2-6 6-3 win over Melinda Czink in the second round of the Citi Open in Washington.
Fourth-seeded American Vania King of America joined Russia's Pavlyuchenkova in the quarter-finals with a 6-2 7-6 (7/5) win over compatriot Irina Falconi.
Another home player, seventh seed Coco Vandeweghe, was a 6-2 6-2 winner over France's Aravane Rezai. King and Vandeweghe now face each other for a place in the semi-finals.
Not all the seeds came through unscathed as number eight Olga Govortsova of Belarus suffered a surprise 1-6 6-0 6-3 defeat at the hands of Eugenie Bouchard.
Govortsova convincingly won the first set and led 3-0 in the decider, but her 18-year-old Canadian opponent battling back both times to claim a triumph.
Other second round winners were Slovakian duo Jana Cepelova and Magdalena Rybarikova.
