A little part of me wants to believe that, during the creation of the original Super Mario Bros, somebody involved in the game thought "one day this character will be popular enough that he'll have his own Tennis-based spin-off and people will be able to play the game we're making now using clever backhand shots."
In all seriousness, this is one of those features that makes you buy a game out of respect for the developers. Whoever had this idea, whoever implemented it, my hat (red, with an M) goes off to you. If you can't watch the video: a fully playable Super Mario Bros using Tennis balls – who would have thought it?
Mario Tennis Open is now available for purchase and, if previous entries in the Mario Tennis series are anything to go by, it'll probably be worth taking a look at.