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Djokovic's right arm troubles him at US Open; Keys wins

Rafael Nadal, of Spain, holds his left wrist while answering questions after defeating Denis Istomin, of Uzbekistan, during the first round of the U.S. Open tennis tournament, Monday, Aug. 29, 2016, in New York. (AP Photo/Alex Brandon)

Tennis star Rafael Nadal

Rafael Nadal pulled out of the London Olympics on Thursday due to injury.