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 he realized during his pre-match warmup that he couldn't run or serve well enough.
"I came out here and tried to warm up 45 minutes or so ago, and it was pretty apparent that my movement was probably 30 percent or 40 percent,'' he said.
"Unfortunately that's just not good enough to get it done at a tournament like this,'' Roddick said. "I didn't feel like I could go out and try to compete and win a tennis match. It's definitely a tough prospect trying to beat Roger with no serve and not being able to move much.
"The risk/reward wasn't there. You're risking further injury where it might cut into preparation for next year.''
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 off-season.
"I don't think we're looking at anything more than a week or so,'' he said. "I was planning on going home anyway. Basically I'm just starting that process four days earlier by not being able to finish here.''
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 finish with two victories.
"I called him this morning at about 10:00 or 10:30 and said, 'You should prepare like you're going to play. I'm probably 50/50 to play tonight, and I'll let you know as soon as I try to warm up,' " Roddick said.
The 26-year-old Roddick has been hampered by injuries this year, twice sitting out a month, the first after retiring from the Rome Masters in early May after just three games with a shoulder injury.
"You don't really want to end your year like that, but in a way I'd like to leave all these things in 2008 and kind of use them all up here as opposed to bringing them into 2009 with me,'' Roddick said.
"Hopefully I have all these nicknack injury things out of way. They've been pretty frustrating. I feel like I've been playing catchup a little bit ... as far as not being prepared and ready. So I'm very much looking forward to getting healthy and actually having some time to try to get fit again and be prepared going forward next year.''
One thing the early departure does is allow him to focus on acquiring a new coach. He split with Jimmy Connors earlier this year, and had since been working with his brother John.
"There's a short list,'' Roddick said without naming any candidates. "I didn't want to start going through the process of talking to people either over the phone or in person until my season was finished. That's something that I'd definitely like to get taken care of in the next couple weeks.''
Roddick, who was making his sixth straight Masters Cup appearance, is a former No. 1 who has been no lower than 12th in the rankings since 2002.
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Roddick withdraws from Masters Cup
