Feedback from the keller-sports.de Product Management:
The toughest tournament in the world is not necessarily played with the hardest balls in the world. This is a good thing, because a Hardcourt is anyway already faster than a clay court, therefore the ball must not be excessively hard and fast. For a hard-court, the ball must endure both in terms of maintaining the pressure as well as the felt. And the Wilson U.S. Open does. The quality of the ball is retained for a considerably long period, especially the felt .
Conclusion: for clay courts they are ok but not first choice, for hard-courts on the other hand they are very good and area very attractively priced alternative.
Since 1978 Wilson has beenthe Official Ball of the toughest tournament in the world, the US Open. Extra Duty felt for longer wear on all court surfaces.