Caroline Wozniacki beats Andrea Petkovic in Stuttgart

Caroline Wozniacki
Wozniacki finished strongly after struggling at the start of the first set

Dane Caroline Wozniacki reached the last four of the Stuttgart Grand Prix with a 6-4 6-1 victory over Germany's Andrea Petkovic.

The world number one was 4-1 down in the first set before rallying to win 10 straight games against her opponent.

Australian Samantha Stosur also reached the semis with a 2-6 6-3 7-6 (7-3) win over world number three Vera Zvonareva.

Stosur will now play Julia Goerges, while Wozniacki will face Agnieszka Radwanska.

Goerges won an all-German battle with a 6-4 6-4 victory over Sabine Lisicki, while Poland's Radwanska defeated another German, Kristina Barrois, 7-5 6-3.

"I am sorry to beat the home favourite but I am happy to be in the semis and hope you will support me," said Wozniacki.

"Today I felt good in here and my volleys felt great, an area I have been working on. I am pleased at the way I was able to finish the match today."

Stosur was also delighted with her own performance.

"It is the best match I have had all year," said last year's French Open finalist. "To come through this close and tight match is just wonderful."

The semi-finals will take place on Saturday as there is no play scheduled in Stuttgart on Friday.