You won't find a better Mario Sports title
Don't expect this to be a dumbed down port of Mario Tennis for the N64; this is an entirely new game. You can play a simple exhibition game with Mario and his pals (you get a lot more characters if you can link it to Mario Tennis on the N64, as indicated on the box.) There's training mini-games, linked play with a friend, and then the main game mode: Mario Tour. In this mode, you name an up-and-coming tennis player who has just enrolled in a Tennis Academy. You are assigned a partner who you also get to name and play doubles with. You then can practice or play ranked matches in different areas of the rather large Academy campus. After you win a match or complete a training game, you are awarded XP points, which you use to upgrade you and your partner and their abilities. The end goal is (obviously) to advance through the ranks of singles and doubles play until you and your partner are kings of the tennis court.
The gameplay in ranked matches is pretty simple, and similar to the N64 version. You have topspin, backspin, lob shots, drop shots and power shots. The game is a bit strategic on when and where you place and time your shots in order to defeat your opponent. The control is much easier then the N64 version, and it takes much less time to really get a feel for it.
This is a game you really need to try for yourself in order to understand just how good it is. This is one of the greatest games I have ever played, and IMO is superior to all the sequels that have come out so far. You can get this game for a very reasonable price off eBay, I suggest you buy it and give it a play, it's a hard game to put down