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Mario Tennis was developed by Camelot Software Planning and published by Nintendo. The game originally came out for the Nintendo 64 in Japan on July 21st, 2000, and in North America on August 28th, 2000.
The game featured a cast of twelve characters, and was notably the first appearance of Waluigi in a Nintendo game. Gameplay consists of pressing one or both of the main buttons on the Nintendo 64 controller to make the ball spin different ways. The characters are delineated by class, with lighter characters having faster speed but less power, and heavier characters having the opposite. The game features a variety of different surfaces which change gameplay. There are also challenge modes in which obstacles appear on the court.
The game has spawned several sequels, including a Gameboy Color version of the original title, Mario Power Tennis (Gamecube, 2004), Mario Tennis: Power Tour (Game Boy Advance, 2005), Mario Tennis Open (3DS, 2012), and Mario Tennis: Ultra Smash (Wii U, 2015).
Mario Tennis Aces
On January 11th, 2018, Nintendo announced Mario Tennis Aces, a Mario Tennis game for the Nintendo Switch to be released in Spring of 2018. The game will feature the first single player story mode in the series since 2005's Mario Tennis: Power Tour.
Mario Tennis Aces was released June 22nd, 2018. The game added several mechanics, including "zone speed," which allows players to use a power meter to slow down gameplay and reach balls; a "zone shot," which is a powerful trick shot players can trigger when their meter is full; and trick shots, which will allow players to quickly race to where the ball will land. The game received a score of 76/100 on Metacritic. Positive reviews praised the game's tight mechanics and addictive gameplay, while many critics were disappointed in the lack of a compelling story mode, calling the single player campaign an extended tutorial. Online, major YouTubers covered the game, including VideoGameDunkey (shown below, top left), Alpharad (shown below, top right), Game Grumps (shown below, bottom left) and ZeRo (shown below, bottom right).
Mario Tennis, particularly the Nintendo 64 version has been widely critically acclaimed. The game has a score of 91/100 on Metacritic. Critics lauded the addictive gameplay and tight physics and controls of the game. Later versions were not as well-received, with the Wii U's Mario Tennis: Ultra Smash being widely recognized as one of the worst Mario Tennis sports games. 
While Mario Tennis itself does not have a wide online fanbase, news about the series is often met with excitement. For example, a thread on /r/NintendoSwitch following the announcement of Mario Tennis Aces generated over 4,500 upvotes.