All about the Wilson US Open Ballen XD 3 st. The Wilson US Open tennis ball is the official match ball of the fourth Grand Slam of the year, the US Open in New York. This ball has been played with in New York for over 25 years. Every year the tournament takes place at the end of August/beginning of September and around 70,000 balls pass through it during the tournament.The ball is gas-filled and made of very high-quality wool and extra durable felt. The good quality of the ball ensures longer durability on all surfaces. The ball is known for its superior playability, consistency and durability. The ball is ITF and KNLTB approved and very suitable for playing in the spring competition and/or open tournaments.