Serena Williams suffered a third set collapse against Karolina Pliskova. Pliskova won 6-4, 4-6, 7-5 .

After leading 5-1 in the final set, Wlliams’ game unraveled. She had four match points but couldn’t convert. Williams , it appeared, rolled her ankle, which affected her movement after she had landed awkwardly in a foot-fault on the first of four match points . Pliskova picked up her game from that point and William could not find hers.

“I did everything I could on those match points,” Williams said. “I can’t say that I choked. [Pliskova] literally played her best tennis ever on those shots. I’ve never seen anything like it. Next time I’m up 5-1 against anybody, I need to make sure I play lights out. It’s a good learning experience.”

“I just felt a chance,” Pliskova said of that moment. “For sure it was in her head. I saw a chance and I just took it.”

Pliskova next plays US Open champion Naomi Osaka, who easily beat Elina Svitolina 6-4, 6-1. In the other semifinal, American Danielle Collins plays Petra Kvitova.