Mecha-Bowser Mayhem is a minigame in Mario Power Tennis. The minigame is played on the Bowser's Castle Court. The objective of the game is to defeat Mecha-Bowser, who is on a rampage. The player has to toss powered-up tennis balls at Mecha-Bowser, and defeat him before Mecha-Bowser defeats the player. However, the player can also use the Bob-ombs and Bullet Bills that he shoots occasionally to deflect back at him. The player is also able to use other power-ups, such as Mushrooms or Stars to assist them in the game. Mushrooms speed the player up, Stars make the player invincible for a short while. It has three difficulties: Aluminum Frame, Iron Frame, and Titanium Frame. The only difference between these difficulties, is that Mecha-Bowser will have higher health in the harder difficulties. Mecha-Bowser may also breathe fire onto the field, which might damage the player.
In the Wii version of the game, the player may not shoot powered-up energy balls until they get a mushroom, which does not speed them up. Also, the game is easier, as Mecha-Bowser's attacks do less damage to the player, and more Bob-Ombs and Bullet Bills appear. Also, if one gets a star, in addition to being invincible, one will be able to shoot more powerful energy balls. Finally, one can charge the energy balls up.