Serena Williams of the U.S. returns a shot to Na Li of China during their tennis match on the second day of the WTA championship in Istanbul, Turkey, Wednesday, Oct. 24, 2012. (AP Photo)

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Serena Williams of the U.S. returns a shot to Na Li of China during their tennis match on the second day of the WTA championship in Istanbul, Turkey, Wednesday, Oct. 24, 2012. (AP Photo)