Third-seeded Danielle Collins of the United States had to stop her match against sixth-seeded Xiyu Wang of China because of a lower back injury. Wang won the first set 6-2 before Collins shook her opponent's hand at the net.

"Sometimes this happens when the weather changes suddenly," Collins said, according to the Austin American-Statesman. "I think that played a role. That's arthritis for you... it creeps up on you sometimes."

Anastasia Sevastova was in front of Slovakia's Anna Karolina Schmiedlova 6-4, 2-1 before falling on a net approach and hurting her left leg. One point later, she retired from the match.

Xiyu Wang's semi-final foe will be top-seeded Anhelina Kalinina of Ukraine, who rallied past fifth-seeded Diane Parry of France 1-6, 6-4, 6-2.

Schmiedlova will face eighth-seeded Yue Yuan, who won an all-Chinese matchup against Yafan Wang 6-3, 7-5.

San Diego Open

Top-seeded Jessica Pegula of the United States downed Russia's Anna Blinkova 6-1, 2-6, 6-2 to reach the semi-finals at San Diego.

Pegula is competing for the first time in a month after missing action due to a neck injury. Her next opponent will be sixth-seeded Marta Kostyuk of Ukraine, who came from behind to defeat fourth-seeded Anastasia Pavlyuchenkova of Russia 3-6, 6-4, 6-3.

The other semi-final will feature third-seeded Emma Navarro of the United States and Katie Boulter of Great Britain. Navarro blitzed Australia's Daria Saville 6-4, 6-2, and Boulter cruised past seventh-seeded Donna Vekic of Croatia 6-4, 6-3.