The world No2 slumped to a 3-6 7-5 6-4 defeat by the 28th seed to set up a clash with Agnieszka Radwanska in the last 16.
“It’s tough to explain. If I knew what happened, I would tell you,” said Kvitova.
“But really I’m not really sure what happened out there. I was up in the second set. Suddenly I felt like she’s coming back, playing a little bit aggressive.
“Suddenly from my side, I didn’t have answer for it. My serve didn’t help me at all this time, as well. I was really struggling with each shot which I played.”
Meanwhile, world No5 Caroline Wozniacki reached the fourth round after beating Camila Giorgi in straight sets.
“It was nice to win this one,” said Wozniacki. “You never really feel comfortable against a player like her.
“She hits the ball as hard as she can. Thankfully for me, it wasn’t really going in that much. I also kept serving well and I returned well, which was a big help for myself today.”
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