Those yellow, green -- or sometimes orange -- felt balls are more complicate than they appear when you pop the can. More than 200 brands of tennis balls are manufactured to the specifications set forth in the Rules of Tennis and approved for play by the International Tennis Federation. Although many types of tennis balls exist, perhaps the most marked distinction is between pressurized and pressureless balls.
Because pressurized tennis balls have a hollow core filled with pressurized gas, they will have more bounce than pressureless balls. This effect is temporary, however, as pressurized tennis balls have a short play life. The gas inside quickly escapes the hollow core, making the ball unplayable. In fact, when removed from the can, a pressurized ball will become useless in two weeks or less, even without use. Pressureless balls gain bounce when used as the felt wears away and the rubber softens.
Pressurized tennis balls also have more spin response than pressureless balls. However, the spin response, like the bounce of the pressurized ball, will decrease with time and use, as the pressurized gas in the core of the ball gradually diffuses through its rubber shell. Pressureless balls become more responsive to spin with use as their exterior felt covering wears away.. Their resilience comes from their denser rubber core and, as the internal pressure of the balls is the same as atmospheric pressure, they don't become flat and unresponsive.
Pressurized balls have less mass than pressureless balls, so they travel faster. However, with age, pressurized balls will slow down, while pressureless balls will only get faster, as the slight drag from their felt covering diminishes with wear. The ITF has approved pressurized balls with decreased pressure for use by beginners, as they will travel slower and bounce less.
Pressureless tennis balls are heavier and harder than pressurized balls, so they impact the racket with more force. This is hard on the racket arm of the person hitting the ball.