The Wilson US-Open tennis ball has the following features: The Wilson US Open tennis ball is the official game ball used during the Grandslam Open in the USA. The gasfilled ball is made of high quality wool and felt. The ball is well known for it’s superior playability and sustainability. Specifications: 4 yellow tennis balls by Wilson ITF approved Suitable for all types of court The US Open is a yearly event that takes place the end of August and is the last Grand Slam tournament of the season. The US Open is held in New York and played on hardcourt. Some of the winners of the US Open are: Roger Federer, Pete Sampras, Kim Clijsters and Chris Evert. Gasfilled balls: Gasfilled balls are under pressure. They often come in a pressurized tin. The ball bounces due to the internal overpressure. Gasfilled tennis balls are usually used by competition players and professionals.