Dubai, United Arab Emirates (Sports Network) - Former Wimbledon champion Petra Kvitova upset defending Dubai titlist Agnieszka Radwanska, while former world No. 1 Caroline Wozniacki was a hard-fought quarterfinal winner Thursday at the $2 million Dubai Tennis Championships.

The Day-4 nightcap saw a sixth-seeded Kvitova cut down the third-seeded Radwanska 6-2, 6-4.

The 2012 Wimbledon runner-up Radwanska beat German Julia Goerges in last year's Dubai finale.

Meanwhile, the eighth-seeded Danish star Wozniacki fought back to beat former Wimbledon runner-up and French wild card Marion Bartoli 4-6, 6-1, 6-4 on the hardcourts at Dubai Tennis Stadium.

Wozniacki was the Dubai champion two years ago and will face the left-handed Czech Kvitova in Friday's semifinals. The two women have split six career matchups, but Kvitova has won three straight in the series.

Fifth-seeded 2012 French Open runner-up Sara Errani of Italy held off Russian Nadia Petrova 6-4, 0-6, 6-3, while Errani's good friend Roberta Vinci ousted seventh-seeded former U.S. Open champion Samantha Stosur 6-2, 6-4. The struggling Stosur misfired for 28 unforced errors in the setback.

Errani and Vinci, who are currently the world's No. 1 doubles team, will square off against each other in Friday's other semi.

The 17th-ranked Vinci said her recent success in doubles has helped with her ever-improving singles play.

"Probably in doubles, you can try also something that you can play in singles," Vinci said. "I probably play more relaxed and then I play better in the singles."

Errani is 4-2 lifetime against Vinci, including wins in their last four meetings.

The Sports Network