Mario Power Tennis is a genuinely fun arcade tennis experience, and in my opinion the crown jewel of the robust collection of GameCube-era Mario sports titles. In addition to characters from the series with wildly differing attributes that encourage varying styles of play, there are enough arcade-y gimmick twists on the formula to keep the game fresh, whether you're playing by yourself or with others.
The actual game of tennis, all gimmicks aside and turned off, is very solid. You can play either singles or double with 1-4 players, and the control scheme is very simple and easy to pick up. Novices can play a decent game with just the left stick and normal topspins via the A button, while more experienced players can take advantage of slices with B, lobs with A->B, and drop shots with B->A, and smashes with A+B. The moveset is small, but mastering the time and place for each of these swings will take some effort. To boot, there are a handful of different types of characters with different strengths on offense and defense. Power characters have strong smashes and great spiking ability but lack mobility in the backcourt. Speed characters are mostly the opposite, with great defensive mobility and poor hitting power. Tricky characters have massive curves on their hits but relatively low stats otherwise, making them a bit gimmicky but powerful in the right hands. Defense characters have long reaches that make up for their slower movement speeds, and make great backline doubles partners. Finally, Technical characters specialize in strong angles which can be difficult to volley. Mario & Luigi, of course, provide all-around characters to get your bearings before you find your playstyle.
The major addition Mario Power Tennis brings to the base formula comes by way of the offensive and defensive Power Shots, which either help the player reach a ball they couldn't get to or increase the power of one they're about to the volley. The largest issue with the Power Shots is the animation time - although only comprising a few seconds each they completely halt the action, and are earned after only a handful of successful normal volleys. In doubles settings, 4 players using their Power Shots in succession can feel chaotic as much as it is tedious. Many of them are pretty creative, although some are certainly better and useful in more situations than others. For example, Daisy's defensive Power Shot has her diving towards the ball, physically moving her to a likely far-from-center position that can be easily exploited on the returning volley. Alternatively, Peach's defensive Power Shot brings the ball back within her reach, not changing her position, which improves her ability to get back into the action without consequences. There are various little quirks like that which arise as you experiment with different characters, and in combination with the character's type and strengths/weaknesses you'll start to see a bit of a hierarchy form. That said, none of these advantages are such that equally skilled players would ever consitently lose given certain matchups (outside of Power characters versus Tricky ones, who never stand a chance in hell). The overall balance is interesting and depending on the opponent you'll quickly surmise what strategies would work best to gain the upper hand, which is great fun, especially in multiplayer where psychological effects are in play as well.
In addition to the regular exhibition mode, there is a series of tournaments to play in both singles and doubles mode, with each successive victory unlocking new characters, courts, modes, and minigames - all of which have a great amount of variety. A huge, avoidable misstep here is that the doubles tournaments are still single-player only, requiring the use of a very unreliable COM partner, which can feel a little frustrating depending on your choice of character. The matches get progressively longer in number of sets as the opponent difficulty is raised, making for a nice challenge. Finally, there are some minigames that can help improve your skills in both single-player and multiplayer settings, though they are fairly limited in their scope outside of one very clever twist on Mario Paint, where you volley globs of paint into a picture until it's colored correctly. They are a fun distraction and there are a lot of them but they're a tiny bit half-baked: a Mario Party-style take on the training exercises from Virtua Tennis 2.
Finally, the courts occasionally have gimmicks to them as well, usually with concepts re-used from other successful Mario-adjacent GameCube titles. I really like the collective look and feel of this Nintendo era, and this sort of continuity is creative and feels fresh. There's a Luigi's Mansion court with ghosts that can block your movement, three Super Mario Sunshine courts with sludge on the ground, moving court bounds, and a tilting hanging court, respectively, a Donkey Kong: Jungle Beat court with snappers that impede your movement, a WarioWare court made of conveyor belts, and a classic Mario Bros-inspired court with enemies from the original arcade game. The coolest part about these courts is that the effects are always triggered by the match participants, and can be used to gain an upper hand. Similarly, if burdened by the gimmick of the stage, the "reset" buttons for the gimmick in question also lie on the stage and can either be walked over or hit on the opponent's side of the court, giving the player the power to manipulate the court in real time. It's frantic and absolutely insane, and definitely where the ingenuity of the formula comes to life the most.
In all, Mario Power Tennis is a pretty outstanding arcade tennis game with some great character quirks and balancing, inspired court design, some interesting twists on the normal tennis gameplay, and fun diversions that make it hard to put down. It isn't perfect - doubles can be chaotic and a little frustrating, some matchups seem downright unwinnable, and outside of the tourneys there's not an enormous amount of content to dig into. The minigames in particular could have been more fun in single-player if they had a larger emphasis on improving the player instead of making a fun party game. In all, though, you'll be hard-pressed to find a better-designed tennis game on the GameCube, and maybe in the entire 6th generation of consoles. Definitely a must-have for basically every audience.
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