SHANGHAI, China — Andy Roddick has withdrawn from the Masters Cup after spraining his right ankle during practice — an injury he said did not appear to be serious.

Roddick made the announcement Wednesday, hours before he was to play Roger Federer in a Red Group match. He was replaced in the tournament by 26th-ranked Radek Stepanek, the first alternate here.

Roddick said he rolled the ankle during a warmup drill for practice Tuesday and initially hoped treatment would allow him to play, but when he warmed up Wednesday afternoon, it was clear he couldn't be competitive.

He estimated his movement at only 30 to 40 percent, and that his powerful serve was particularly affected.

"Unfortunately that's just not good enough to get it done at a tournament like this," Roddick said. "It's definitely a tough prospect trying to beat Roger with no serve and not being able to move much."

Roddick said his trainer and a doctor told him the injury didn't appear to be serious, and he anticipated being able to do his planned training during the offseason.

"I don't think we're looking at anything more than a week or so," he said.

Roddick lost his opening round-robin match at the season-ending tournament to Britain's Andy Murray, and Federer fell to Gilles Simon.

Stepanek, of the Czech Republic, could qualify for the semifinals but would have to win both of his matches and hope no more than one of the other players in his group finishes with two victories.