There is an interesting thing to note. In Elusive Shift, Jon Peterson makes and excellent case for how early D&D was shaped by players who came in from the wargaming side and those who came in from the SF Fandom side. I noted that a lot of my generation of designers were from the wargaming side of fandom. However, at the very beginning, we had a strong component of established SF/Fantasy authors who were contributing to the milieu. authors such as Fritz Leiber, Gardner Fox, Harry Fischer and L Sprague DeCamp were not only helping create the fantasy genre we mined in the game, but were also creating new games in the early TSR years.
In addition, we have early games like the original Dungeon! (which predates D&D) and M.A.R. Barker's War of the Wizards. War of the Wizards is a game set in Tekumel, Barker's setting for Empire of the Petal Throne. The games were released pretty close to each other, WotW name-drops a lot of the places, creatures, and background color of the setting. It never mentions Tekumel itself by name, but rather the nation of Tsolyanu and refers to the RPG as "Petal Throne". The gods are the Lords of Glory and the Rulers of the Shadows and generally identified as good and evil, or law against chaos. Creatures include a lot EPT standards, along with more traditional names that you can use instead.
The concept of the game is very simple - two spellcasters (choice of sorcerer or priest) are at opposite ends of a football field. They cast spells at each other, which then advance down the field at their opponents. Spells can affect other spells, walls, and summoned creature, and ultimately the opposing wizard, who has so many hit points. Kill the opposing wizard, and you win.
The first version of this game was self-published by Barker and is apparently rarer than hen's teeth. The "second edition" was in a plastic bag from TSR, and was quickly replaced by one of TSR's early boxes. I played the game itself back in college (Purdue University) in the Boilermaker Grill in the basement of Tarkington Hall. I got hold of the box version is one of TSR's frequent clean-out-the-library purges. So opening the box, I found I had versions 2 and 3, both with unpunched counters, along with the ruleset for the 1982 Car Wars minigame and instructions on how to paint the Bear Chariot miniature from Ral Partha.
War of the Wizards was a classic "wizards duel" game, and, like Empire of the Petal Throne, was extremely advanced for its time. Released in 1975, it showed that early TSR had a strong commitment to Barker's work. A set of miniature rules followed, and Zenopus Archives has an article on a Dungeon! variant that was in the works that never saw the light of day. Adventure Modules were not much of a thing yet, so there were none from TSR at the time. Yet TSR showed a commitment to Tekumel, and then they didn't, and things faded away by the time the G-series and later the campaign worlds showed up.
It is an interesting part of history, and a good game as well. I'm not sure if this is the first "wizards' duel" sort of game, but it is one of the most involved and detailed. Someone could easily resurrect it, or create a version online (and it is sort of like Plants Vs. Zombies in its approach - things keep changing and staying the same).