Dunlop ATP Championship High Altitude Extra Duty 24 Cans Dunlop ATP Championship Extra Duty tennis balls are officially endorsed by the ATP World Tour. These mid-range tennis balls are ideal for club- and recreational-level tennis players, and the Max Core and Durafelt HD Cloth are durable enough for hard-court play. 3 balls per can. Suitable for play on hard courts. Approved by the USTA and ITF. Important Note: These tennis balls are designed for use in high altitude locations over 3,000 feet. They will not work as well and seem dead or flat in other locations.