Doha, Qatar (SportsNetwork.com) - Top seed Petra Kvitova was upset by ninth-seeded Carla Suarez Navarro at the Qatar Open tennis event.

Also on Thursday, a battle of former world No. 1s saw Victoria Azarenka whip Caroline Wozniacki and another upset came when seventh-seeded Venus Williams upended No. 4 seed Agnieszka Radwanska in quarterfinal action.

The nifty Spaniard Suarez Navarro came from behind to bounce the Czech left-hander and two-time Wimbledon champion Kvitova, 3-6, 6-0, 6-3.

Meanwhile, the 2012 and 2013 Doha champion Azarenka drilled the third-seeded Wozniacki 6-3, 6-1. Azarenka, performing as a wild card this week, also bested Wozniacki in a second-round match at the Australian Open last month.

Azarenka is now 5-4 lifetime versus the Danish Wozniacki, including wins now in their last three meetings.

The 2014 U.S. Open runner-up Wozniacki was the Doha runner-up in 2011.

The former top-ranked star Williams took down the former Wimbledon runner-up Radwanska 6-4, 1-6, 6-3 on the hardcourts at Khalifa International Tennis Complex.

"I just tried to stay focused because she wasn't going to let up," Williams said after the 2-hour, 16-minute victory.

Radwanska also lost to the resurgent Williams in the fourth round last month at the Aussie Open.

The 34-year-old Williams will meet Azarenka in Friday's semifinals. The seven- time Grand Slam champ Williams is 4-0 lifetime against the two-time major titlist from Belarus.

Another quarterfinal result saw Czech Lucie Safarova vanquish sixth-seeded German star Andrea Petkovic 6-2, 6-1. Safarova and Suarez Navarro will collide in Friday's other semifinal.

The 2015 Doha titlist will claim $124,000.