Key Biscayne - Andrea Petkovic became the first semi-finalist at the Miami Open when she beat Karolina Pliskova 6-4, 6-2 on Tuesday.
Petkovic, seeded ninth, arrived at Key Biscayne with a modest 7-6 record this year but has yet to lose a set.
Her opponent Thursday will be the winner of the quarter-final on Tuesday night between three-time champion Venus Williams and Carla Suarez Navarro.
Tomas Berdych advanced to the men's quarter-finals when Gael Monfils fell and hurt his hip.
The No. 8-seeded Berdych was leading 6-3, 3-2 when Monfils retired.
Monfils, seeded 17th, said he bruised his hip but hopes to be able to play in his next scheduled tournament in Houston.
Unseeded Dominic Thiem of Austria advanced by beating No. 28 Adrian Mannarino 7-6 (5), 4-6, 7-5.