The grunt is technically a strong exhale that can be physically gratifying for the tennis player. The grunt is a psychological advantage in that some players feel they hit the ball harder when they grunt. It also serves as a powerful distraction.
Grunting muffles the noise of the racket hitting the ball and keeps the opponent from hearing how hard the ball was struck. The ball is moving so fast, that the opponent will hear the hit before they see it. Grunting makes it nearly impossible to gauge the hit properly. It isn’t illegal to grunt, but the opponent can complain to the umpire about it.
A study revealed the accuracy of the complaint that grunting can distract enough to affect performance. Scott Sinnett, assistant psychological professor at the University of Hawaii at Manoa asked 33 undergrad tennis players to view videos of a tennis shot being hit toward them and quickly judge whether the ball would go to the left or to the right.
After measuring their responses and accuracy, he asked them to do them same, but he covered up the hitting noise with a grunt. The students’ time and accuracy significantly decreased when they heard the grunts.