The woolen "fuzz" on tennis balls creates wind resistance as the ball flies through the air, slowing the ball down considerably, explains Tennis SET. Without such fuzz, the ball moves too quickly and bounces too high for a proper tennis match.
Rabi Mehta, a NASA expert on aerodynamics, called this "fuzz drag." As the fuzz gets worn down and the ball gains a more uniform shape, the ball travels faster through the air and bounces with more force off both rackets and the surface of the court. Upon first glance, a tennis ball appears to be a solid sphere with a smooth surface. However, upon closer examination, it is revealed that the surface of a tennis ball is uneven.