(Updates after later matches)
By Matt Smith
DUBAI Feb 21 (Reuters) - Czech Petra Kvitova defeated titleholder Agnieszka Radwanska 6-2 6-4 in the Dubai Championships on Thursday to set up a semi-final against Caroline Wozniacki.
Italian Roberta Vinci will play compatriot and doubles partner Sara Errani in the other semi after prolonging Samantha Stosur's woes this season with 6-2 6-4 win over the seventh seed.
Kvitova, the 2011 Wimbledon champion, had not won a match during two previous appearances in Dubai but despite producing 38 unforced errors in the tie she proved too powerful for her Polish opponent.
"I'm glad I can be here and I've played three great matches so far," sixth seed Kvitova said in a courtside interview. "It can help me for the rest of the season."
Of her next opponent Wozniacki, who saw off France's Marion Bartoli 4-6 6-1 6-4, Kvitova said: "I saw her play today against Marion and it was a good match. I'm expecting the same.
"It'll be long rallies and I'll have to play my aggressive game like today."
Australian Stosur's win-loss record this year stands at 5-7 following an error-strewn performance against Vinci under the searing desert sun that left her frequently yelling in frustration.
Vinci, 30, broke in the opening game and was always in control against a player who has failed to win a title since triumphing at the U.S. Open in September 2011.
The Italian has now beaten top-10 ranked players in successive matches - she defeated Angelique Kerber on Wednesday - and has yet to drop a set in Dubai.
World number nine Stosur towered over the diminutive Vinci, but was guilty of trying to be too aggressive, clubbing a succession of backhand returns into the net.
Vinci, who with partner Errani are the top-ranked doubles pairing, is a percentage player, relying more on her opponent's mistakes than going for winners herself and Stosur's approach played into the Italian's hands.
Serving at 4-2 down, Stosur ran to the net but Vinci, world number 17, played an angled backhand dropshot winner for a break point.
The Italian duly converted after Stosur, backpedalling from the net, hit a smash long. Stosur then surrendered the set with another errant backhand.
The Australian changed tactics in the second set, slowing the game down and matching Vinci's backhand slice but Stosur was frequently left stranded in mid-court, either arriving too late to block off her opponent's passing shots or coming forward too soon and allowing herself to be lobbed.
In the second match of the day, Errani stormed into a 5-1 first-set lead as she proved more agile around the court than Nadia Petrova, but the world number 12 responded, breaking back after the Italian scuffed a forehand.
The Russian closed to 5-4, before Errani took the set as Petrova came forward to attack the second serve but could only find the net.
Petrova, 30, had the power, Errani the finesse, and it appeared brute force would triumph as the Russian won nine out of 10 games that included a 6-0 second set whitewash.
But Errani, playing with a heavily strapped left thigh and grunting with every shot, swapped breaks early in the deciding set before breaking again to win 6-4 0-6 6-3. (Additional reporting by Alison Wildey in London; Editing by John Mehaffey)