Derek Chisora beat Joe Joyce via unanimous decision in London on Saturday in a remarkable upset victory following 10 gruelling rounds between two British rivals.
Joyce was knocked down in the ninth round after taking on a barrage of punches throughout the fight he was largely expected to win.
The result leaves Joyce's career at a crossroads after three defeats in his last four bouts.
Chisora, clearly keen for a brawl, came out swinging from the opening bell, but Joyce absorbed everything and continued to come forward.
The inevitable gripping slugfest everyone had anticipated was soon in motion.
By the fourth round as his shots kept coming, the crowd was well and truly on Chisora's back, willing him to bite down on his gum shield and continue. He of course duly obliged.
Whenever it looked as though Joyce may be starting to gain some control, Chisora, 40, came out swinging again and landed clean punch after clean punch.
He proved a tricky target for Joyce, but he was clearly fatigued by round five. The toll was clear as he collapsed onto his stool at the end of round seven, gasping for air.
Chisora was cheered on by undisputed heavyweight champion Oleksandr Usyk, who was barking advice and encouragement from ringside at one stage.
In round nine, the arena stormed to its feet as Joyce was knocked down.
Chisora would have been grateful for the breather more than anything. His experience showed as he danced around the ring to see out the remainder of the bout and score an incredible victory.
In his 10th and potentially final fight at The O2 Arena, there was a warm affection from the London crowd at the end for Chisora, a cult hero of British boxing, as he closes in on 50 professional fights.
Joyce insisted after the fight he will continue, but this was seen as a must win by many.
Chisora, meanwhile, will bask in the glory of his win for some time to come, and will almost certainly want more after another spirited victory.
"I knew he [Joyce] was tough, but Jesus Christ, that guy is tough," Chisora told his post fight news conference.
"For a couple rounds, I was moving back my legs were just all over the place. But, you know, it was just two big guys in there bro. The whole point is to stay up and just keep moving.
"It was beautiful and tough at the same time. Yeah, it was hard."
Asked what Usyk was telling him during the fight, Chisora said: "He was saying 'move, move, move,' but everybody was telling me to move, bro. I wanted to move but my legs didn't want to move. They were like 'nah mother-----.'"
Promoter Frank Warren hailed the efforts of both men and said he'd never seen anything like the clash before.
"So were you entertained or what tonight?" Warren asked his post-fight news conference.
"I didn't think it would go to distance. I thought that obviously the favorite was Joe. [But] you know, it takes two gladiators to create a fight like that. Both of them, you've got to take your hat off to them.
"I thought it was the last two rounds which decided it in Derek's favour, it was a very close fight, but amazing. Look at their ages ... I don't know how Derek does it, I don't know how Joe does it.
"They did not miss at each other at times. Every punch they were throwing was connecting."
Asked what would be next for Joyce after another defeat, Warren said: "It's too soon [to say]. I've said that I felt the loser in this fight would have nowhere to go. But I never predicted or thought it would be a fight like this. I thought it would be competitive, but I didn't think it would be like this million years."