From the Super Mario Wiki
The Clay Court is a tennis court that appears in the Mario Tennis (series). In Mario Tennis (GBC) and Mario Tennis: Power Tour it is where the senior-ranked players play. Alex, Nina, Clay and Ace also play here to try to advance their way to varsity-ranked. Its ground is an orange-like color in all games. It has a slow speed and weak bounce, making a harder court to play on. Defensive or fast characters will be put to a slight advantage due to their statistics.
Mario Power Tennis
playing a singles match on the Clay Court.
In Mario Power Tennis, the clay court appears in Peach Dome. When chosing this court, it is mentioned that the ball travels slowly due to weak bounces off the court.
Mario Tennis Open
In Mario Tennis Open, the clay court appears in Mario Stadium. Like the other clay courts, it has a weak bounce with slow ball speed. It is described as being 'A Court of High Drama'.