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BRISBANE, AUSTRALIA - APRIL 06: Jack Sock of the USA plays a backhand during practice for the Davis Cup World Group Quarterfinals between Australia and the USA at Pat Rafter Arena on April 6, 2017 in Brisbane, Australia. (Photo by Bradley Kanaris/Getty Images)

BRISBANE, AUSTRALIA - APRIL 06:  Jack Sock of the USA plays a backhand during practice for the Davis Cup World Group Quarterfinals between Australia and the USA at Pat Rafter Arena on April 6, 2017 in Brisbane, Australia.  (Photo by Bradley Kanaris/Getty Images)