The tournament where the hero fights in disguise and claims to have been busy is a fairy tale commonplace (such as in The Golden Crab or The Magician's Horse, or in Little Johnny Sheep-Dung and The Hairy Man, where it is actual battle), and from there passed into such romances as Robert the Devil, Sir Gowther, and Lanzelet.
There is also speculation as to its relationship to the German Lanzelet by Ulrich von Zatzikhoven, which features the Queen's abduction but not her affair with Lancelot, and may derive from a version of the story that predates Knight of the Cart.