Wilson US Open Ball x4balls is the official ball of the US Open since 1978. The ball is made of high-grade extra duty felt that offers great performance on hard courts. The high-altitude core gives durability on clay, grass, or indoor courts. It has a weight, height, length, and width of 200gm, 8 cm, 21 cm, and 8 cm respectively. The ball is approved by USTA and ITF for competitive play.