Pro Penn Marathon is Penn’s longest lasting tennis ball featuring exclusive Encore™ Technology for a 22% longer lasting core and high tenacity LongPlay® Felt for extended play. Pro Penn Marathon is the official ball of PTR because of its guaranteed longevity, durability and visibility. This is the Pro Penn Marathon Extra Duty Felt for hard courts. Professional tennis balls are the most technologically advanced tennis ball providing players with top of the line core and felt technologies that allow the ball to play at its best for longer periods of time.