Pro Penn Marathon Xtra Duty Tennis Balls 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. Pro Penn Marathon Extra-Duty Felt is geared for hard courts. Penn’s Professional tennis balls are the most technologically advanced tennis ball providing player with top of the line core and felt technologies that allow the ball to play at its best for longer periods of time.