Fourth seed Victoria Azarenka joined the list of Madrid Open withdrawals with a back injury ruling her out.
The 26-year-old had been scheduled to play Louisa Chirico of the United States in round three on Wednesday, but pulled out during the warm-ups.
Belarusian Azarenka said she will have medical tests before deciding whether to play in Rome before the French Open, which starts on 22 May.
Her departure means the top five seeds are out of the Spanish tournament.
Agnieszka Radwanska, Angelique Kerber and Garbine Muguruza exited in previous rounds while fifth seed Petra Kvitova lost 6-3 6-4 to Australia's Daria Gavrilova on Wednesday.