ALL COURTS: While regular duty tennis balls are best suited for soft, clay courts, they can also be played on all indoor and outdoor hard court surfaces as well. OFFICIAL BALL: Penn Championship is the Official Ball of USTA Leagues, so you know you're playing with high quality and consistent ball. Available in extra-duty felt, regular duty felt ...
https://www.amazon.com/Penn-Championship-Tennis-Balls-Pressurized/dp/B07TVGWBD6