Brazilian qualifier Beatriz Haddad Maia produced a stunning upset as she defeated top seed Sloane Stephens in straight sets at the Mexico Open.
An early break from Stephens put her 1-0 up at the start of the second-round match, but Haddad Maia immediately recovered to level the set before edging ahead at 4-2.
At the start of the second set, it was the Brazilian's turn to take an early break, and she continued the momentum to clinch a convincing 6-3 6-3 victory on her second match point.
Joining her in the quarter-finals in Acapulco is former world number one Victoria Azarenka, who dismissed Germany's Tatjana Maria 6-2 6-1.
The Belarusian, currently ranked number 50 in the world, hit 20 winners and broke six times to triumph in just 84 minutes.
Azarenka will next face number five seed Sofia Kenin, who progressed after Britain's Katie Boulter was forced to retire from their second-round match.
Kenin had been leading 6-4 4-1.
Number three seed Donna Vekic awaits, after the Croatian swept past Christina McHale 6-4 7-5.
There were also wins for Zheng Saisai, who battled from a set down to overcome Timea Babos 4-6 6-4 6-3, and Bianca Andreescu, who prevailed 6-2 7-5 over Romania's Mihaela Buzarnescu.
Wang Yafan claimed her quarter-final spot after Monica Puig retired at 4-1 down.
