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.
Mario Power Tennis
In Mario Power Tennis, the grass court is one of the three courts in Peach Dome. The ball speed is fast while the bounce is weak. The standard mode is the default mode, but players can also play in ring match or item battles. The Star Cup and Planet Cup tournaments take place in the Grass Court.
Mario Tennis Open
Mario Tennis Open: Tennis aces face off in this famous court.