Sam Stosur's French Open preparations were handed a welcome boost with a hard-fought victory over second seed Vera Zvonareva at the Porsche Tennis Grand Prix in Stuttgart

23522 - Stosur sinks Zvonareva to reach Stuttgart semis

The Australian No. 1, who slipped down the rankings after failing to defend her Family Circle Cup title in Charleston, looked set to exit the tournament at the quarter-final stage after losing the opening set, but produced an impressive comeback to seal a 2-6 6-3 7-6(3) victory.

A runner-up at Roland Garros last year, Stosur will face another stiff test in the semi-finals with the Stuttgart crowd guaranteed to be firmly behind her opponent Julia Goerges, who came out on top of an all-German quarter-final against Sabine Lisicki 6-4 6-4.

23525 - Stosur sinks Zvonareva to reach Stuttgart semis

Stosur, who lost in the final to Justine Henin last year, has now beaten Zvonareva in their last six meetings. It is the first time she has reached the semi-finals of a WTA Tour event in 2011, calling the victory her "best match of the year".

Caroline Wozniacki continues to remain untroubled in Stuttgart, easing to another straight-sets victory over Andrea Petkovic. Wozniacki dropped just three games in her second round match, and she allowed Petkovic only five games in the quarter-finals. The world No. 1 broke five times en route to booking a semi-final place with a 6-4 6-1 triumph.

Wozniacki will play Agnieszka Radwanska in the last four after the Pole overcame Kristina Barrois 7-5 6-3. Radwanska was strong behind her first serve, dropping just 13 points as she advanced in exactly two hours.

Meanwhile, at the Grand Prix de SAR, second seed Yaroslava Shvedova continued a tournament of shocks by losing 6-2 7-6(5) to Anastasia Pivovarova. Shvedova's defeat comes on the back of top seed Aravane Rezai's departure on Wednesday. Elsewhere, Alberta Brianti eased past Ksenia Pervak 7-5 6-2.