The Official Ball of the US Open Every September the toughest tournament in the world uses over 70,000 of the toughest tennis balls: Wilson U.S. Open balls. Since 1979, Wilson has been the only ball that has held the distinction of being the "Official Ball of the U.S. Open". Constructed of premium high-grade wool that is woven into a thicker, denser felt, the Wilson U.S. Open tennis ball is known for its superior playability, consistency and durability. 1 case of tennis balls = 24 cans.