Petra Kvitova withdrew from the St. Petersburg Ladies Trophy in Russia due to illness on Thursday.
The No. 3 seed was set to face Russia's Ekaterina Alexandrova in the quarterfinals, but instead was forced to pull out of the competition.
"I'm really sorry to withdraw from the St. Petersburg Ladies' Trophy due to illness," Kvitova said in a statement.
"I wasn't feeling well during my match yesterday and hoped I would feel better today, but unfortunately that's not the case."
The withdrawal means that Alexandrova advances into the semifinals of a WTA competition for the second time this season.