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.

"I do a lot of that a lot on my serve also, so it's definitely a tough prospect trying to beat Roger with no serve and not being able to move much."

Roddick, who is playing the season-wrapup event in China for the third straight year, leaves with a 3-5 record at the venue.

"It's unfortunate for this event, but I don't think we're looking at anything more than a week or so," Roddick said.

"In the grand scheme of things (that) is good news considering the nature of the injury. Unfortunately it's bad news for me participating here." (dpa)