(AP Photo/Charles Krupa). Andy Murray, of the United Kingdom, reacts after a shot against Jo-Wilfried Tsonga, of France, during the fourth round of the 2014 U.S.
(AP Photo/John Minchillo). Novak Djokovic, of Serbia, serves against Philipp Kohlschreiber, of Germany, during the fourth round of the 2014 U.S.
(AP Photo/Jason DeCrow). Kei Nishikori, of Japan, reacts after defeating Milos Raonic, of Canada, during the fourth round of the U.S.
(AP Photo/John Minchillo). Serena Williams, left, has her angle re-taped as Venus Williams looks on during a break between games during a quarterfinals doubles match against Elena Vesnina and Ekaterina Makarova at the 2014 U.S.
(AP Photo/Darron Cummings). Victoria Azarenka, of Belarus, reacts after defeating Aleksandra Krunic, of Serbia, during the fourth round of the 2014 U.S.