From the Super Mario Wiki
The Grass Court is a tennis court that appears in the Mario Tennis series. It always has a fast ball speed with a weak bounce. It appears in the Game Boy Color game Mario Tennis and Game Boy Advance game, Mario Tennis: Power Tour as the court used in the Island Open. After beating the Island Open, Alex, Nina, Clay and Ace move on to the Peach Tournament. In all the games, it is a light green court made of grass. There are a numerous amount of seats due to the crowd wanting to watch tennis on this famous court.
This court appears in the Nintendo 64 game Mario Tennis, where it has a weak bounce with fast shots.
Mario Power Tennis
In Mario Power Tennis, the grass court is one of the 3 courts in Peach Dome. When playing on this court, it is mentioned that the tennis ball travels fast, but the bounces are weak.
Mario Tennis Open
The Grass Court in Mario Tennis Open
In Mario Tennis Open, the grass court appears as one of the courses in Mario Stadium.
Mario Tennis Open: Tennis aces face off in this famous court.