Jack Sock

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United States' Jack Sock tosses the ball for a serve to Switzerland's Marco Chiudinelli during a Davis Cup tennis match Friday, Feb. 3, 2017, in Birmingham, Ala. (AP Photo/Butch Dill)

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United States' Jack Sock slides to return a hit from Switzerland's Marco Chiudinelli during a Davis Cup tennis match Friday, Feb. 3, 2017, in Birmingham, Ala. (AP Photo/Butch Dill)

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Jack Sock, of the U.S, returns the ball to Serbia's Dusan Lajovic during their third round match of the French Open tennis tournament at the Roland Garros stadium, in Paris, France, Saturday, May 31, 2014. (AP Photo/Darko Vojinovic)

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Serbia's Dusan Lajovic serves the ball to Jack Sock, of the U.S, during their third round match of the French Open tennis tournament at the Roland Garros stadium, in Paris, France, Saturday, May 31, 2014. (AP Photo/Darko Vojinovic)

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Jack Sock, of the U.S, slams a forehand to Serbia's Dusan Lajovic during their third round match of the French Open tennis tournament at the Roland Garros stadium, in Paris, France, Saturday, May 31, 2014. (AP Photo/Darko Vojinovic)