Roddick, ranked No. 6, has withdrawn from the eight-man end of season tournament which begins in London on Sunday, The Times of London and Spanish sports newspaper Marca said on their Web sites on Tuesday.
Roddick has been struggling to overcome the knee injury that forced him to pull out of his match against Stanislas Wawrinka in the opening round of the Shanghai Masters last month.
Robin Soderling, the Swede who reached this year's French Open final and will have a year-end ranking inside the top 10 for the first time, is expected to replace him and Jo-Wilfried Tsonga of France will travel as the first reserve, The Times reported.
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