From the Super Mario Wiki
The Clay Court is a tennis court in the Game Boy Advance game, Mario Tennis: Power Tour. It is where the senior-ranked players play. Clay and Ace also play here to try to advance their way to varsity-ranked. Its ground is an orange-like color with a surrounding green fence.
 Mario Power Tennis
In Mario Power Tennis, when playing on this court, it is mentioned that the ball travels slowly due to weak bounces off the court.