Top seed Daria Gavrilova advanced to the San Antonio Open quarter-finals after a hard-fought clash against Greece's Maria Sakkari on Wednesday.
Australian Open quarter-finalist Gavrilova was in big trouble after losing the first set and seeing the second set forced to a tie-break, but eventually prevailed 4-6 7-6 (7-5) 6-3 in an error-strewn match in which she struck nine double faults.
Gavrilova will take on Alison Riske in the quarters, after the American knocked out seventh seed Kirsten Flipkens 7-5 6-1.
The only other seed to make the last eight was Japan's Misaki Doi, who got the better of Xinyun Han courtesy of two tie-breakers, winning the match 7-6 (7-5) 7-6 (7-4).
Doi will face Samantha Crawford, who got the better of Carina Witthoeft 6-1 6-3.
Ana Konjuh will meet Anna-Lena Friedsam after they defeated Magda Linette and Catherine Bellis respectively, while Donna Vekic will play Tsvetana Pironkova after their wins over Kiki Bertens and Lauren Davis.