ST PETERSBURG, Russia (REUTERS) - Donna Vekic stunned Australian Open runner-up Petra Kvitova 6-4 6-1 in the quarter-finals of the St Petersburg Ladies Trophy on Friday to (Feb 1) claim the biggest victory of her career.

World number 30 Vekic took full advantage of the 42 unforced errors committed by the Czech top seed and defending champion to prevail in 83 minutes.

Vekic next faces the winner of the all-Russian clash between third seed Daria Kasatkina and wildcard Vera Zvonareva in the semi-finals.

Second-ranked Kvitova, who was beaten by Japan's Naomi Osaka in the Australian Open final last week, had her service broken for a second time in the opening set as Vekic went ahead 5-4 before claiming the early advantage in the contest.

Kvitova took the first game of the second set but Croatia's Vekic raised her level a few notches, reeling off six games in a row to seal her third victory over a top-five player.

Earlier, fourth seed Aryna Sabalenka celebrated her first week in the world's top 10 with a comfortable win, defeating Russian qualifier Ekaterina Alexandrova 6-3 6-4.

Up next for the world number 10 from Belarus is Kiki Bertens, who battled past Russia's Anastasia Pavlyuchenkova 6-2 3-6 6-0.