Czech fifth seed Petra Kvitova overcame a stubborn display from China's world number 90 Wang Yafan to win 7-5 6-3 and make the US Open last 32.
Kvitova, 28, broke late to seal the opener after letting a 4-1 lead slip.
Wang's consistent groundstrokes and a solid serve offered more resistance in the second set, before she finally folding in an hour and 46 minutes.
Kvitova next plays 20-year-old Belarusian Aryna Sabalenka, who won the New Haven title earlier in August.
Sabalenka beat 2010 runner-up Vera Zvonareva 6-3 7-6 (9-7) to make the last 32.