(Adds details of Errani win)
By Matt Smith
DUBAI Feb 21 (Reuters) - Samantha Stosur's season of woe continued at the Dubai Championships when she crashed out in the quarter-finals following a 6-2 6-4 defeat by Italian Roberta Vinci on Thursday.
Vinci will now play doubles partner Sara Errani in an all-Italian semi-final after her compatriot beat Russia's Nadia Petrova 6-4 0-6 6-3.
Stosur has lost more matches than she has won this year, with her win-loss record standing at 5-7 after she produced an error-strewn performance 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 she 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 No.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 broke for the first time early in the second set and also changed tactics, slowing the game down and matching Vinci's backhand slice.
But the Italian responded with an immediate break back, taking Stosur to deuce before the latter struck successive forehands wide.
Stosur was frequently left stranded in mid-court, either arriving too late to block off Vinci's passing shots or coming forward too soon and allowing herself to be lobbed.
The Italian broke again for a 4-3 lead after another Stosur error and served out the match.
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 the taller 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, refused to cave in.
The 2012 French Open finalist swapped breaks early in the deciding set before breaking again for a lead she would not relinquish.
In Friday's remaining quarter-finals, former world number one Caroline Wozniacki faces France's Marion Bartoli and 2011 Wimbledon champion Petra Kvitova plays Agnieszka Radwanska. (Reporting by Matt Smith, editing by Pritha Sarkar)