Defending champion Agnieszka Radwanska overcame Eugenie Bouchard at the Connecticut Open, while Peng Shuai also reached the quarter-finals.
Radwanska, the 2016 winner of the WTA Premier event in New Haven, beat wildcard Bouchard 6-3 7-5 on Tuesday.
Top seed Radwanska improved to a 4-0 win-loss record over Bouchard after managing five breaks of serve in the second-round clash.
Awaiting the Pole in the quarter-finals is Peng, the eighth seed getting past Croatian veteran Mirjana Lucic-Baroni 6-2 6-3.
Australian Daria Gavrilova booked her spot in the last eight with a hard-fought 7-5 7-6 (8-6) win over Timea Babos.
Qualifier Elise Mertens also progressed, claiming a 6-2 7-5 victory over Daria Kasatkina.
Meanwhile, in a first-round clash, Carla Suarez Navarro led Jana Cepelova 6-4 4-0 when the Slovakian qualifier retired.