Unseeded Kazakh Zarina Diyas caused an upset at the Citi Open, shocking Lesia Tsurenko for a place in the quarter-finals.
After eliminating Wimbledon sensation Coco Gauff in the round of 32, Diyas upstaged fifth seed Tsurenko 6-4 6-4 on Thursday.
Diyas broke Tsurenko five times throughout the one hour, 53-minute contest in Washington, where both players required medical timeouts in muggy conditions.
Diyas – who lost to Tsurenko in Eastbourne in June – will next face Camila Giorgi for a spot in the semi-finals of the WTA International event.
Giorgi dropped the first set, but the Italian rallied as she overcame Rebecca Peterson 3-6 6-3 6-4.
Fourth seed Hsieh Su-wei recovered from a slow start to beat Varvara Gracheva 7-5 4-6 7-6 (8-6) in Thursday's final match.
Hsieh trailed 5-2 in the opening set but she reeled off five consecutive games to move ahead in the match before Gracheva levelled it up. However, the former was not to be denied in a third-set tie-break.
Awaiting Hsieh is American wildcard Caty McNally, who prevailed 6-3 1-6 6-3 against Christina McHale.