From the Super Mario Wiki
Baby Mario Target Shot is a minigame in Mario Tennis for the Game Boy Color. The player controls Baby Mario and the gameplay involves hitting the ball into the target area on the opposite side of the court, indicated by the flashing corners. The Ball Bot returns the shots. Flat Shots, Strong Topspins, and Strong Slices give 3 points while Smashes, Lobs, and Drop Shots give 5 points. Bonus points can be earned by hitting the shot indicated onscreen, and they depend on the type of shot. The game ends once Baby Mario fails to hit the ball into the target.