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.