Sweden's Robin Soderling will replace Andy Roddick in the ATP World Tour finals after the American pulled out with a knee injury on Tuesday.
"I'm really disappointed ... I love playing in London and I've heard so many good things about the venue," Roddick told the ATP website.
"However, I have not fully recovered from my knee injury and I won't be able to compete. One of my goals in 2010 will be to qualify for this event again," added the 27-year-old, who also pulled out of last year's tournament in China due to injury.
The finals start at London's O2 Arena on Sunday.
World number six Roddick, this year's losing Wimbledon finalist, also pulled out of the Paris Masters earlier in the month after injuring his knee in Shanghai in October.
He has not played since then although he returned to light practice this week.
Soderling was the first alternate for the world's biggest indoor tennis tournament.
The big-hitting Swede reached his first grand slam final at the French Open this year after also becoming the first player to beat Rafael Nadal at Roland Garros.
The draw for the finals will be held later today.