Stephens stunned in quarters as Tsurenko keeps title defence alive
Top seed and US Open champion Sloane Stephens was upstaged by world number 183 Stefanie Voegele, who surprisingly reached the Mexican Open semi-finals.
Voegele played above her ranking as she stunned American star Stephens 6-4 5-7 6-2 in Acapulco on Thursday.
The 27-year-old Swiss - seeking her first WTA Tour-level final appearance - traded breaks with 2016 Acapulco champion Stephens in the opening set before claiming the decisive break in the final game.
Stephens responded in the second despite surrendering a 3-1 lead as she reeled off three successive games to force a deciding set.
But Voegele was not to be denied her first top-20 victory since 2013, dropping just two points in four games to book a final-four berth thanks to her fourth win in five matches against Stephens.
Next up is Rebecca Peterson, who crushed fourth seed Zhang Shuai 6-2 6-1.
Defending champion Lesia Tsurenko cruised through to the semis after blitzing second seed Kristina Mladenovic.
In a rematch of last year's decider, Ukrainian seventh seed Tsurenko was too good in a 6-2 6-2 victory on the Mexican hard courts.
Standing in the way of Tsurenko and back-to-back appearances in the final is Australia's Daria Gavrilova.
Gavrilova - the third seed - outlasted Veronica Cepede Royg 6-4 4-6 6-2.