US Open champion Sloane Stephens suffered an upset in the Mexico Open after being dumped out of the tournament by world No 183 Stefanie Voegele.
The top seed appeared sluggish against Switzerland's Voegele, who claimed her biggest win in five years with a 6-4 5-7 6-2 victory.
Stephens started the quarter-final proceedings with errors on her backhand, giving Voegele the opportunity to break in the final game of the first set.
The American showed more aggression in the second, winning the last three games in a row to take the set, but she faltered in the decider as the Swiss dropped just two points in four games.
Voegele will now face qualifier Rebecca Peterson, who beat fourth seed Zhang Shuai 6-2 6-1 to advance to the semi-final.
Elsewhere in Acapulco, No 3 seed Daria Gavrilova beat Veronica Cepede Royg 6-4 4-6 6-2, while defending champion Lesia Tsurenko overcame No 2 seed Kristina Mladenovic in straight sets, 6-2 6-2, in a replay of last year's final.