Top seed Sloane Stephens rallied to reach the WTA Mexican Open quarter-finals, while defending champion Lesia Tsurenko joined the American in the last eight.
US Open champion Stephens advanced in Acapulco thanks to Wednesday's 5-7 7-6 (8-6) 6-0 victory over Dutch qualifier Arantxa Rus.
Stephens – winner of the WTA International tournament in 2016 – claimed her first victory since winning last year's US Open on Tuesday.
And Stephens was forced to dig deep midweek, saving a match point in the second-set tie-break to prevail after two hours, 26 minutes.
Next up for Stephens is Stefanie Voegele, who defeated Maria Sakkari 6-3 6-3.
Tsurenko's title defence remains on track after she eliminated Australian Arina Rodionova 6-3 6-1.
The Ukrainian seventh seed will face Kristina Mladenovic in the quarters.
Frenchwoman Mladenovic – the second seed – outlasted Beatriz Haddad Maia 3-6 6-3 7-5.
Third seed Daria Gavrilova survived a scare to overcome wildcard Renata Zarazua 7-5 6-3, with Veronica Cepede Royg awaiting in the next round.
Royg was too good for fifth seed Irina-Camelia Begu 7-6 (7-3) 6-1, Monica Puig lost 6-1 6-4 to Rebecca Peterson, while Zhang Shuai saw off Jil Teichmann 7-6 (7-5) 6-4.