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)
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)
Serbia's Dusan Lajovic clenches his fist as he playsJack 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)
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)