There isn't too much to Mario Tennis Open. Eschewing the over-the-top sports titles of the past, this title instead goes for a fairly mundane style of play, with only some minor changes to a typical tennis game to spice things up, like the ability to play as your Mii in addition to the typical retinue of Mario characters. Problem is, it's not really enough to create a title that's anything more than mildly enjoyable, if not quite short.
The gameplay is much more subdued than other Nintendo sports titles. You hit the ball back and forth across the court, and occasionally a shot slips past the opponent. There are special 'Chance Shot' areas for you to shoot from, areas on the court that light up when your opponent makes a bad rebound: using the right shot from these areas does gives your shot a little more force behind it, but that's really it.
The controls are a peculiar bunch here. Various areas of the touch screen map to the different kinds of shots that you can make, and for the most part, they make sense: a higher area of the screen gives your ball some topspin, the center will do a basic shot, and so forth. Now, thankfully, you can just use the face buttons on the 3DS to do these, but they are mapped strangely, where X does a basic shot, A performs a topspin shot, and two buttons are required for particular shots. It's weird, but can be gotten used to with some time.
There are also two modes of view in the game, a top-down view, and another that is activated by holding the 3DS vertically. In this mode, your character moves automatically, and you just control when to swing the racket while pointing the 3DS at where you want the ball to go. It's essentially 'very easy' mode, making it damn near impossible to give up more than a couple points in an entire tournament. Makes things very dull, too.
The bulk of Mario Tennis Open's gameplay comes from the single player modes. There are a number of tournaments to play through (both singles and doubles), exhibition matches if you are just looking for a quick game, and special modes. These special mini-games let you do such things as shooting through rings for points, trying to keep the ball from falling into a black hole, or - and this appears to be everyone's favourite - playing against scrolling mario levels, using the ball to hit goombas, grab coins, and break blocks. This last mode is really the most enjoyable of the bunch, but that's not saying much.
Playing these special games earns you coins, which allow you to purchase clothing and accessories that are unlocked by playing matches in tournaments and exhibitions. It's the only customization in the game, as well as the only presence of progress through game mechanics you can see (don't expect any RPG mechanics or such in this Mario Tennis title). It's lacking, and doubly so since the additions to your character subtly change your stats, but it is damn hard to compare two pieces of clothing using the display you're given. It isn't really worth it to try, and instead just equip whatever you find looks best.
And there's multiplayer. Not much to say here: you can play with your friends. There's also the ability to get other people's Mii via streetpass and play against them.
Mario Tennis Open feels like a step back (except when it comes to visuals) from previous Mario Tennis games. There are less modes than previous titles, no RPG mode from the Game Boy Color version, and the gameplay itself is a lot more basic. Though the quick matches are alright for some quick pick-up-and-play gaming, there isn't enough depth to the title to last for more than five minutes at a time. So, unless you're really, really aching for a portable Mario Tennis title, this game isn't worth the love.