Mattek-Sands, a wild-card entrant, defeated the second-seeded Hsieh 7-5, 6-2. Hsieh, because of earlier weather delays, was playing her third match in two days.
Hsieh had an early first-set break but was unable to keep the lead and Mattek-Sands took the set after her third break. Mattek-Sands jumped ahead in the second set and then stopped four break-point chances by Hsieh in advancing to the semifinals.
She'll next face third-seeded Anastasia Pavlyuchenkova, who was a 6-2, 6-0 winner over Ashleigh Barty. Pavlyuchenkova piled up five breaks and faced just one break point. She lost only 12 points in the second set.
No. 4-seeded Ayumi Morita rallied for a 6-7 (7-9), 6-4, 6-1 decision over Luksika Kumkhum. Morita suffered a pair of service breaks in the first set but only one after that. She took 69 percent -- 80 percent on serve -- of the points in the third set.
Karolina Pliskova defeated Patricia Mayr-Achleitner 7-5, 6-1, pounding out 11 aces and winning two-thirds of the points when serving. Pliskova also won half the points in receiving serve.