Now, tennis balls are fuzzy because the fuzz helps them slow down. When a tennis ball moves through air, every fibre of the fuzz acts like an obstacle for air. This increases the drag, thus slowing it down.
Besides this, the fuzz also increases the spin.
Imagine a ball spinning in clockwise direction. Now, above the ball, the direction of air is same as the direction of rotation. However, below the ball, it is opposite. This opposing force combined with the fuzz increase the drag below the ball. Hence, the pressure below becomes much higher than it is above. This pressure difference imparts a lift, thus increasing the spin.