Fans of the Virtual Boy will be glad to know of the work which has been done to make Mario Tennis playable by multiplayer once more:
Like numerous other Virtual Boy titles, Mario's Tennis was going to support the GameLink Cable to offer multiplayer modes, a fact that was even mentioned in an early In-Store promotional video. After the GameLink Cable got cancelled, naturally none of the commercially released games contained any type of simultaneous multiplayer mode.
It was long believed that Mario's Tennis did not even contain any leftover multiplayer code - until M.K. went to great lengths to find and restore exactly that! He just released a patch that restores 2 of 3 game modes (P1 vs P2 and P1/COM vs P2/COM) to a fully working state. The third mode (P1/P2 vs COM/COM) is buggy and won't be fixed until any Virtual Boy emulator implements full link cable emulation to shorten debugging and testing times to a healthy amount. While Reality Boy does offer network-based link cable emulation, it lacks support for the respective interrupts, which Mario's Tennis' multiplayer modes depend on.
Check out the release thread for a brief summary of the discovery and restoration process as well as the patch download.
If you're eager to play some Mario's Tennis multiplayer now and got a friend with another Virtual Boy and flash cart nearby, you might be interested in Kevin Mellot's fantastic homemade link cable