World number one Victoria Azarenka admitted 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.
Her father Zoran was left with his head in his hands while Germany's Fed Cup coach Barbara Rittner was shocked after seeing the injury.
"What a terrible thing to have happened, I feel so very sorry for her," said Azarenka.
"She is a great player and I hope she will recover soon."
It looks like Petkovic has suffered another serious injury setback having 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.
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 clay-court tournament as 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.
"This court contributed to Mona's game as she is very tall and can play very fast, so it was very hard for me to break her rhythm. I am sure the match between us would look very different on a hard court."
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 either ex-world number one Caroline Wozniacki or Germany's Angelique Kerber.