Wilson US Open Extra Duty Case of 24 Sanctioned for ITF and USTA tournaments, the Wilson US Open Extra Duty Case of 24 comes with 24 tins of three-ball tins. The extra duty balls provide optimal performance and durability on clay/indoor courts. Since 1979, Wilson has been the only ball that has held the distinction of being the "Official Ball of the U.S. Open". The balls are constructed of premium high-grade wool that is woven into a thicker, denser felt providing superb playability, durability and consistency. Wilson Us Open is made with premium tex/tech industries felt. Extra Duty gives the optimal performance and durability on hard courts.