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.