Play like the pros with the Wilson Extra Duty 4 Ball Tube tennis ball, the official ball of the US Open, which is designed for longlasting play on all hard courts. The Wilson Extra Duty 4 Ball Tube tennis balls are USTA and ITF approved for competitive play. They are made with premium textech industries felt for extra durability and are the official ball of the US Open. USTA and ITF approved. Made with premium textech industries felt. Extra Duty. Optimal performance and durability on hard courts. Ideal for longer play on hard court surfaces. US Open official ball. The Wilson US Open tennis balls are made with premium Tex/Tech Industries felt and ensure optimal performance and durability on hard courts, as well as on clay/indoor courts. They feature a high altitude core for excellent performance on courts above 3500ft. The balls are supplied in a 4 ball can.