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
Mario Tennis Open
Mario Tennis Open: Tennis aces face off in this famous court.