From the Super Mario Wiki
Super Mario Tennis is one of the four special games in Mario Tennis Open. The game plays like a tennis version of Super Mario Bros., with the player hitting the tennis ball at enemies, coins, etc. The goal of the game is to get to the Flagpole at the end of each level by hitting it with the ball. Baby Mario can be unlocked by completing level 1-3 in this game. Additionally, a Wario costume can be unlocked by completing level 1-4 with at least 800 coins, and a Waluigi one can be unlocked by completing level 2-4 with at least 800 coins. This and the other levels of World 2 are only accessible by playing with another person via local multiplayer.