These Wilson US Open tennis balls feature an extra duty design with a longer wool nap and a true balance of fluff and durability that is ideal for hard courts. Sold in cases of 72 balls (24 cans per case), the US Open tennis balls are Wilson's fastest and lightest playing ball and are made for the advanced player. The ball's 100% natural rubber core provides maximum consistency and resiliency. These US Open tennis balls have been the official ball of US Open tennis since 1978.