ST. PETERSBURG, Russia -- No.8 seed Donna Vekic of Croatia pulled off a stunner at the St. Petersburg Ladies Trophy on Friday, upsetting No.1 seed and defending champion Petra Kvitova, 6-4, 6-1, to enter the semifinals of the Premier-level event.

22-year-old Vekic, ranked World No.30, shocked the World No.2 in one hour and 23 minutes, racking up the final six games of the match. This is Vekic’s best win by ranking of her career, and her third win over a Top 5 player overall. Vekic also snapped Kvitova’s six-match undefeated streak at St. Petersburg.

"My game [plan] was to try to be the first one to be aggressive, to open up the court, and try to make her run and defend, and obviously serve well," Vekic told the press, following her signature victory.

Vekic has moved up over 20 spots in the WTA rankings over the last 12 months. "I’ve been working hard, doing the right things," said the Croatian. "When you work hard, it pays off, and you’ll be successful on court, and I think in the last year, it’s all started to come together for me. I started the year well, and hopefully I can continue it well."

Czech star Kvitova, twice a Wimbledon champion, had won the lone prior meeting between the two players in straight sets, at Miami in 2014. But the 2019 Australian Open runner-up was undone by 42 unforced errors. Vekic had a handle on Kvitova's serve throughout, converting five of her nine break points.

Kvitova was broken in the first game of the match, but the Czech was able to strike back directly, blasting a beautiful forehand passing shot down the line for a clean winner to level the first set at 1-1.

But that would be the only instance where Vekic would be troubled on serve throughout the early stages of opener. The Croatian tracked down drop shots and returned them for winners, and stayed solid on return when she could get a crack at Kvitova’s serve, keeping pace with the Australian Open finalist through 4-4.

Vekic broke through in that game, as Kvitova squandered a 40-15 lead with three errors and a double fault, her second of the game, to hand over the 5-4 lead and allow Vekic to serve for the set. Vekic had to survive one final break point, but she did so with aplomb, slamming a forehand winner off a weakly-returned drop shot, and then held on to clinch the opening frame.

In the first game of the second set, Kvitova erased a break point with a forehand winner, eventually taking the game and a 1-0 lead. But that would signal the last gasp for an increasingly weary Kvitova, as Vekic dominated the remainder of the set.

The unforced errors mounted for the Czech as Vekic scrambled all over the court, prolonging points until either Vekic slammed a screaming winner or Kvitova blasted a mistake. Vekic reeled off five games in a row to reach 5-1, culminating with a searing forehand winner to close out that game.

Kvitova nearly stopped the streak in the next game, as one match point fell by the wayside when a Vekic rally backhand clipped the net and fell back on her side. But after three more deuces, Vekic reached a second match point with a barrage of ferocious forehands. There, a Kvitova backhand flew long and Vekic was rewarded with her first Top 2 win.

Vekic will have to deal with a partisan crowd in the semifinals, as she will face wild card Vera Zvonareva, who emerged as the winner of the all-Russian quarterfinal rumble with No.3 seed Daria Kasatkina. Vekic lost her only previous match with Zvonareva, in the second round of Wimbledon in 2014.