US Open champion Sloane Stephens suffered her ninth defeat in 11 matches as she was beaten by world number 183 Stefanie Vogele in the Mexican Open.
World number 13 Stephens, 24, began the tournament without a win since her maiden Grand Slam triumph in September.
After victories in the first two rounds, the American top seed slipped to a 6-4 5-7 6-2 quarter-final defeat.
Swiss Vogele’s win was her first over a top-20 player since she beat Stephens in 2013.
The 27-year-old faces Sweden’s Rebecca Peterson, who beat fourth seed Zhang Shuai of China 6-2 6-1, in the semi-finals.
Defending champion Lesia Tsurenko of Ukraine will play Australian third seed Daria Gavrilova in the other semi.
In the men’s draw, Argentine Juan Martin del Potro beat Austria’s Dominic Thiem 6-2 7-6 (9-7) to set up a semi-final against German second seed Alexander Zverev, who overcame American Ryan Harrison 6-4 6-1.
South African Kevin Anderson, the fifth seed, faces American Jared Donaldson for a place in Saturday’s final.