Shanghai - Andy Roddick withdrew from the Masters Cup shortly before his match on Wednesday, citing a right ankle injury he suffered during training the day before.
The American, who lost his opening match to Andy Murray, will be replaced in the Red group by Czech Radek Stepanek.
The American dropped his bombshell a few hours before he was due to take to the court against Roger Federer.
"In practise I just went over on it, we tried as much as we could to get it better," he said. "I can't really stop and start very quickly. It's anything when I go kind of over on the outside it hurts.