Defending champion Petra Kvitova has been forced to withdraw from the WTA Championships in Istanbul with a virus.
The Czech, 22, looked well below-par as she slipped to an emphatic 6-3 6-2 defeat by Agnieszka Radwanska in her first group match on Tuesday.
It has subsequently emerged that the 2011 Wimbledon champion was struggling with illness and she confirmed on Wednesday that she had pulled out of the tournament.
"I am on my way to the airport now to fly home and see my doctor. I am so sad to withdraw from the #WTA Champs and to leave Istanbul," she tweeted.
"I want to say sorry to the Turkish fans who supported me so much here. I just wasn't able to compete at 100 per cent and need to recover."
First alternate Sam Stosur will now take Kvitova's place for the remaining White Group round-robin matches against Maria Sharapova and Sara Errani.
Kvitova revealed she is still hoping to compete for her country when Czech Republic defend their Fed Cup title against Serbia next week.
"Will keep fingers crossed for Fed Cup and let you know. Good luck to Sam, hope she can win some matches for me," she added.