Dunlop ATP High Altitude Extra Duty 24 Cans The Dunlop ATP High Altitude Extra-Duty 24 Can case features tennis balls officially endorsed by the ATP World Tour. These tennis balls are specifically designed for playing at high elevation. 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.