The only help I can offer is what's already on my Mario Tennis codes page:
If you have an N64 Transfer Pak and the N64 version of Mario Tennis, you can unlock some more characters and mini-games. Slap your GBC Mario Tennis in the Transfer Pak, put it in your controller, and then turn on the N64. A "Transfer" option should appear in the menu. After going to the Transfer screen and doing the Game Pak Check, choose "N64 characters," which will upload Bowser, DK, Waluigi, Wario, and Yoshi to your GBC game. Along with the 5 characters comes their mini-games also.