Why do people say Sharapova is grunting during her tennis matches?

Maybe I have a different dictionary to everybody else, but why do tennis journalists say Sharapova is 'grunting' when she's playing tennis? The definition of a grunt is a hollow sound made by a hog. To utter or express with a deep guttural sound. Sharapova and Co shriek, scream, screech, wail, but they do not grunt.