LOS ANGELES, CA - DECEMBER 13: Kei Nishikori in action against Jack Sock at the Maria Sharapova and Friends, presented by Porsche exhibition on December 13, 2015 in Los Angeles, California. (Photo by Joe Scarnici/Getty Images for IMG)
LOS ANGELES, CA - DECEMBER 13: Kei Nishikori in action against Jack Sock at the Maria Sharapova and Friends, presented by Porsche exhibition on December 13, 2015 in Los Angeles, California. (Photo by Joe Scarnici/Getty Images for IMG)