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BEIJING, CHINA - OCTOBER 07: Jack Sock of the United States returns a ball against Andreas Haider-Maurer of Austria during the day five of the 2015 China Open at the China National Tennis Center on October 7, 2015 in Beijing, China. (Photo by Emmanuel Wong/Getty Images)

BEIJING, CHINA - OCTOBER 07:  Jack Sock of the United States returns a ball against Andreas Haider-Maurer of Austria during the day five of the 2015 China Open at the China National Tennis Center  on October 7, 2015 in Beijing, China.  (Photo by Emmanuel Wong/Getty Images)