From the Super Mario Wiki
The Grass Court is a tennis court from the Game Boy Advance game, Mario Tennis: Power Tour. This court is where the Island Open matches are held. After Clay and Ace play here, they move on to the Peach Tournament. It's a light green court with seats surrounding it. There are a numerous amount of seats due to the crowd at the Island Open.
Mario Power Tennis
In Mario Power Tennis, when playing on this court, it is mentioned that the tennis ball travels fast, but the bounces are weak.