For three solid hours they joust. Sir Launcelot and Sir Gawaine, hammering one another out in the field in front of Castle Benwick. Their horses die in the first pass, but without a word they dismount, pull out their swords, and just go to town on one another.
Three hours in, as noon approaches, Sir Launcelot starts to wonder what’s up. Sir Gawaine is not a guy he would have expected to put up this solid a fight. Maybe he’s just motivated to expend hidden reserves, what with avenging his family. Or maybe — and this is the true thing — Sir Gawaine has a secret.
This came up once before, roundabout Chapter 17 of Book IV. From around ten to noon, Gawaine’s strength multiplies. For a while at noon, he is straight-up super-strong. So while, normally, Gawaine would slowly be getting weaker and weaker, as Launcelot bruises and nicks him, instead he comes at Launcelot with more and more power!
Is this the untimely end of Launcelot? No, because even when he’s up against a guy with super-strength, Launcelot still fights smart. Malory breathlessly reports that Launcelot switches to a purely defensive fighting style as Gawaine’s strength waxes. He’s able to wait out Gawaine-as-Hourman, your standard Foreman-Ali matchup.
Finally, Gawaine’s strength leaves him. Where this magic strength came from, Malory can’t say. Malory blames an holy man for it, which I would interpret as Nacien, except that it first shows up way way way back when Gawaine’s a young man. It’s completely inexplicable there, too. Regardless, once Gawaine’s burned out, he becomes easy pickings for Launcelot, who smashes his skull in.
Gawaine lies on the ground, exhausted and bloodied and half-dead. As Launcelot turns away, however, Gawaine wheezes out “Why withdrawest thou thee?”
“I won,” says Launcelot.
“No, you haven’t won yet. I’m still alive. Long as I’m alive, this fight isn’t over. Get back here and finish it, or next time we meet, I will!”
Launcelot shakes his head no. “Wit thou well, Sir Gawaine, I will never smite a felled knight.” He walks away from the still-protesting Gawaine, as doctors and medics and leech-handlers rush in to treat the half-dead knight.
As Launcelot walks back to his castle, none of Arthur’s army dares to touch him. He passes King Arthur’s tent on the way, and tells his old friend and former liege lord to just go home because there’s nothing Arthur’s going to accomplish here.