Really great thing about the scene, isn't just the great conflict between the characters, but how it directly relates to their characters. Roy's facing a man he knows is a superior fighter, and he's in really bad shape, but he would never let Cunningham get away with it. And even facing terrible odds, he accepts the fight. Plus, even though it would him to cheat somehow, and the guy he's fighting is absolute scum, he still fights fair.
Cunningham meanwhile, jumps at the chance to kill Roy. And mainly because of his sadism. He'd love to get rid of his enemy, and he resents Rob for having defied him, but it's mainly about being able to cruelly kill someone. Similarly, he could have try to have Rob killed, or cheat, but his desire to personally humiliate and kill Rob, and his expectation to do so, means he accepts a fair fight. because he sees as a chance to inflict one more cruelty on Rob. And to that purpose, even though he could beat Rob quickly and easily, because of his sadistic personality, he toys with him. Makes him suffer. And tries to further humiliate him. But, because he doesn't just kill him, he leaves Rob alive, which means Rob has a chance (whereas otherwise he was as good as dead).
And here also Cunningham underestimates Rob. He can't imagine Rob's resilience - because he's not that sort of guy. And so at the end, when Rob's on his knees, he can't imagine there's any way Rob could keep fighting, let alone win. Plus, going back to how he toyed with him, he ends up placing Rob in a position where he thinks he's harmless. He inadvertently gives Rob an advantage.
And finally, Rob shows his almost superhuman will. His incredible strength. And his absolute refusal to accept defeat. Even when he's beaten, he knows the fight's not over.
And here's the real strength of the scene. Not only does this fight perfectly reflect the men's characters, but the villain loses because of his character, and the hero wins because of his character. And on that note - how many movies have the hero winning simply because of skill, and luck? Here he wins because of cunning, and resilience. But more importantly, because of who he is