Samantha Stosur out in Dubai quarters Sam Stosur crashes out of the Dubai Open in the quarter-finals after a loss to unseeded Roberta Vinci. Samantha Stosur plays a forehand return in her loss to Italy's Roberta Vinci Source: AP SAMANTHA Stosur lost to doubles specialist Roberta Vinci of Italy 6-2 6-4 overnight in the quarter-finals of the Dubai Championships. Stosur's loss follows a quarter-final exit in Doha last week as well as disappointing second round defeat at the Australian Open. The 2011 US Open champion made 28 unforced errors and was broken twice in each set. The Australian's first serve percentage was only 58 in the second set and she faced eight break points. "Way too many errors," Stosur said. She did well to not be bothered by anything I was trying to do. She used her slice very well which kept all the points neutral. I made way too many mistakes on those neutral balls. Despite the latest setback, the 28-year-old said she felt she was improving over her dismal start to the year. January didn't really go as well as I wanted it to. I wasn't playing up to the standard and level that I think I'm capable of," she said. The last couple of weeks, it's gotten better. I've played many more good matches than bad so it's certainly on the way up. Vinci will face doubles partner Sara Errani in the semi-finals. The seventh-ranked Italian beat 12th-ranked Nadia Petrova of Russia 6-4 0-6 6-3. "I spoke with my coach to play more aggressive," said Vinci, who beat sixth-ranked Angelique Kerber to reach the quarter-finals. I played a lot on her backhand. She has an unbelievable forehand. So on my mind, I try to play against her backhand because she has a lot of problem with backhand, and try to change some slice and also some top when I have time. Former world No1 Caroline Wozniacki reached the semi-finals with a 4-6 6-1 6-4 win over Marion Bartoli, shrugging off a code violation in the process. Wozniacki was warned for illegal coaching as the match boiled up to a thrilling finish, apparently because she was talking to her father Piotr. Wozniacki will tackle Petra Kvitova for a place in the final after the 2011 Wimbledon champion underlined her new status as the unofficial tournament favourite by ending the title defence of Agnieszka Radwanska. The Czech player again showed she is in her best form since winning her only grand slam title 19 months ago, out-hitting and out-serving the third-seeded Pole 6-2 6-4.