I was reading an article about knights and misconceptions about them and came across certain claims such as:

These modern expectations exist because they have precedent in medieval literature. The opening of Sir Gawain and the Green Knight—the decapitation of the Green Knight (he got better) by the noble Sir Gawain—takes place in King Arthur’s feast hall. In Thomas Malory’s fifteenth-century Arthurian tales (Le Morte d’Arthur), Arthur’s court is the setting for countless duels, brawls, and other acts of violence. In Beowulf, the titular hero’s fight with the villainous Grendel rages through Heorot—the imposing mead hall of King Hroðgar...

But the thing is, these knightly and heroic tales are clearly fiction. They are medievalist tropes used by medieval authors, but they are still medievalist tropes. Malory and the other later medieval authors gleefully imposed the ideas and ideologies of their own time on the earlier medieval periods they described. They used popular ideas of the early medieval to tell their stories. Actual events were an afterthought.

Is there any historical precedence for such battles? Are they entirely the invention of later authors?