New tennis balls are pressurized and have more pressure inside the ball than outside. That's what gives them their bounce. ... And remember, much a like an opened soda can that goes flat, an opened tennis can will lose their bounce as well, even if they are unused.

How long until tennis balls go flat?

So how long do tennis balls really last? Tennis balls will go bad after about 2 weeks or 3-4 playing sessions. Unopened tennis balls are kept in a pressurized tube to help them retain bounciness and firmness, but even those will expire after two years (due to very tiny leaks).

How do you keep tennis balls from going flat?

How to keep tennis balls pressurized? Keeping the tennis balls unopened is the best way to keep them pressurized. Once opened, tennis balls will start to lose their pressure. To keep used tennis balls pressurized, make sure to keep them in a pressurized storage container at room temperatures.