World No.1 Caroline Wozniacki suffered her first defeat since lifting the Australian Open trophy when she was edged out 7-6 (7-2) 6-3 by Russia's Daria Kasatkina in the quarter-finals at the St Petersburg Ladies' Trophy.
The 20-year-old Kasatkina was dominant in the tiebreak to claim the opening set and won three of the last four games of the match against Wozniacki, who had fought hard to get back on serve halfway into the second set.
"She just won the Australian Open, and since I beat her, it means I was playing really well," Kasatkina said. "I was just trying to play point-by-point and focus on the rally."
It was Kasatkina's second career win over a reigning world No.1, having beaten Angelique Kerber when the German held the top ranking in Sydney last year.
World No.23 Kasatkina will next face defending champion Kristina Mladenovic, who continued her strong run with a 6-4 6-3 win over Czech Katerina Siniakova in the quarter-finals.
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Earlier in the day, two-time Wimbledon winner Petra Kvitova booked her place in the semi-finals with an attacking masterclass to defeat second seed Jelena Ostapenko 6-0 6-2.
Kvitova, who entered the tournament as a wildcard, broke her Latvian opponent's serve five times to seal the most one-sided victory in the pair's five head-to-head meetings.
"At the end of the last match, I found my rhythm a little bit - I knew I had to play a little bit better today, against someone who is playing very aggressive," Kvitova said on-court.
"I had to stay focused on every point. It was just about a few games which I was serving and kept my serve... I served pretty well today, so it was great."
Kvitova's will next face fifth seed Julia Goerges, who recorded a convincing 6-3 6-3 win over Russian qualifier Elena Rybakina.
