Stuttgart, April 27

World number one Victoria Azarenka said she was sorry to reach the quarter-finals of Stuttgart’s WTA tournament on Thursday as Germany’s Andrea Petkovic exited with a serious injury.

Azarenka took the first set 6-2, but with the scores tied at four-all in the second, Petkovic shouted in pain after twisting her right ankle and was in great distress as she was helped off in their second-round tie.

"What a terrible thing to have happened, I feel so very sorry for her," said Azarenka, who helped Petkovic off court.

"She is a great player and I hope she will recover soon."

Petkovic only made her return to tennis in Germany’s Fed Cup defeat to Australia last weekend after three months out with a back injury. She also tore her ACL knee ligament in 2008.

"My ankle swelled up as soon as I twisted as it. We are checking the ligaments for damage. I am icing it tonight and then tomorrow I will a couple of scans," said Petkovic.

"I was looking for my rhythm at the beginning of the match and just as I was beginning to find it I suffered the injury. It is definitely unlucky, but I will be back again."

In Friday’s last eight, Azarenka will now play wildcard Mona Barthel, ranked 35th in the world, who earlier claimed her second scalp of the claycourt tournament when she knocked out seventh seed Marion Bartoli.

"That was one of my best matches," beamed Barthel. "There is a natural extra motivation when the stadium is full and the crowd is behind you, but I am keeping my feet on the ground."

The 21-year-old German, who hit 11 aces to out-muscle ex-world number one Ana Ivanovic on Wednesday in straight sets, needed just 71 minutes to beat France’s Bartoli 6-3, 6-1 as she hit a further seven aces.

"My opponent had nothing to lose, that makes a first time head-to-head difficult," said Bartoli of Barthel, who is among a batch of promising German female tennis players and won the Hobart WTA tournament in January.

"Mona played a lot tougher than I thought. I watched her playing Ana and had noticed that she has a hard serve and strong strokes. "But I was still surprised about how fast she could play, I virtually had no chance to defend, especially on this slippery surface, at times it left like I was playing on ice."

US Open winner Samantha Stosur of Australia will face world number two Maria Sharapova in her quarter-final after beating defending champion Julia Goerges.

In a repeat of last year’s semi-final, Stosur, 28, was taken to three sets by Germany’s Goerges in their second-round clash before the Australian dug deep for a 6-2, 2-6, 6-3 victory.

There were no such problems for French Open winner Li Na, the eighth seed, who beat Akgul Amanmuradova of Uzbekistan 6-4, 6-4 to book a quarter-final with Poland’s Agnieszka Radwanska.

Wimbledon champion Petra Kvitova had few problems in her 6-2, 6-2 win over Italy’s Francesca Schiavone to set up a last eight match against Germany’s Angelique Kerber, who enjoyed a 6-1, 6-2 win over sixth seed Caroline Wozniacki, the former world number one.