Australian Open finalist Petra Kvitova struggled with her serve and was eliminated from the St. Petersburg Ladies Trophy on Friday after losing to eighth-seeded Donna Vekic 6-4, 6-1 in the quarterfinals.

The second-ranked Kvitova landed only 48 percent of her first serves, double-faulted six times and was broken five times.

Kvitova, the defending St. Petersburg champion who lost to Naomi Osaka in the Australian Open final, was playing in her 14th match in 24 days. It was Vekic’s fifth match in that time.

Vekic will next face either third-seeded Daria Kasatkina or Vera Zvonareva in the semifinals.

Fourth-seeded Aryna Sabalenka reached the semifinals by beating Russian qualifier Ekaterina Alexandrova 6-3, 6-4. She will next play either Kiki Bertens or Anastasia Pavlyuchenkova.

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Petra Kvitova of Czech Republic returns the ball to Donna Vekic of Croatia during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Petra Kvitova of Czech Republic returns the ball to Donna Vekic of Croatia during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Donna Vekic of Croatia serves during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match against Petra Kvitova of Czech Republic in St. (AP Photo/Dmitri Lovetsky)
Donna Vekic of Croatia serves during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match against Petra Kvitova of Czech Republic in St. (AP Photo/Dmitri Lovetsky)
Donna Vekic of Croatia returns the ball to Petra Kvitova of Czech Republic during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Donna Vekic of Croatia returns the ball to Petra Kvitova of Czech Republic during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Aryna Sabalenka of Belarus returns the ball to Ekaterina Alexandrova of Russia during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Aryna Sabalenka of Belarus returns the ball to Ekaterina Alexandrova of Russia during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Ekaterina Alexandrova of Russia returns the ball to Aryna Sabalenka of Belarus during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)
Ekaterina Alexandrova of Russia returns the ball to Aryna Sabalenka of Belarus during the St. Petersburg Ladies Trophy-2019 tennis tournament quarter final match in St. (AP Photo/Dmitri Lovetsky)