Third-seeded German Alexander Zverev reached his first ATP semi-final of the season on Thursday, beating Ryan Harrison 6-4, 6-1 to set up a clash with Juan Martin del Potro in Acapulco. World number five Zverev, 20, is the highest-ranked player left in the draw of the hardcourt tournament that saw top seed Rafael Nadal withdraw with nagging hip trouble. In the semis he'll face former US Open champion del Potro, who fended off a second-set challenge from third-seeded Austrian Dominic Thiem to triumph 6-2, 7-6 (9/7).
"I was very happy with how I played," said Zverev, who saved all four break points he faced. "I'm starting to play very well again, and that's all that matters for me."
Argentina's del Potro saved three set points against Thiem in the second-set tiebreaker, firing two aces to give himself a match point and taking the contest when Thiem double-faulted.
In the other semi-final, fifth-seeded South African Kevin Anderson will take on American Jared Donaldson.
Anderson defeated South Korean Chung Hyeon 7-6 (7/5), -4 while Donaldson beat Spain's Feliciano Lopez 6-3, 6-1.
Anderson saved six of the seven break points he faced to reach his third semi-final of the season.
"I felt the first set was really touch and go," Anderson said. "I had to save quite a few break points. I felt I played some of my best tennis down on those break points."
In women's action, top-seeded American Sloane Stephens and second-seeded Kristina Mladenovic of France were both sent packing in the quarter-finals.
Switzerland's Stefanie Vogele, ranked 183rd in the world, stunned reigning US Open champion Stephens 6-4, 5-7, 6-2.
Seventh seed and defending champion Lesia Tsurenko of Ukraine made short work of Mladenovic, 6-2, 6-2, in a rematch of last year's final.
Vogele next faces Sweden's Rebecca Peterson, who ousted fourth seeded Zhang Shuai of China 6-2, 6-1. Tsurenko will take on third-seeded Australian Daria Gavrilova, who beat Veronica Cepede of Paraguay 6-4, 4-6, 6-2.