"It was very close [to stoppage]," said a battered but unbroken Calver afterwards.
"The ref wanted to call it off, the doctor said 'have a home town fight, let it go'. I said 'please let me go'."
The decision was made to allow Calver to continue if his cornermen could stem the blood flow between rounds and the cut did not open up any further.
But Calver knew he was now on borrowed time, facing a Palmerston North slugger who had a bullseye to aim at.
"I just had to suck it up and keep going.
"Just having to try, he kept coming in."
Campbell started winging shots at Calver's head, who reached down and scrambled to keep his pursuer at a distance, freeing himself to score with some torrid right and left hooks.
In his eagerness to close the distance, the determined Campbell began walking into shots as Calver's wound did not sap his strength but spurred him on.
A turning point came late in the third round as a tired Campbell lurched out of a neutral corner and straight into a barrage of punches that set the Wanganui War Memorial crowd roaring.
Calver's burst continued into the fourth round as he momentarily buckled Campbell with a leg kick then unloaded with a series of blows.
But it was the visitor who now proved his courage, still standing there even after that blast was followed by three Calver knees from the clinch, all right on the button.
Even Calver couldn't believe it.
"I hit you with my hardest hits, that most people can feel through the [training] pad, and you still kept coming at me, like 'go away'."
Both men were exhausted by the final round, falling forward with their blows, as Campbell continued to close the gap and mash brutal elbows into Calver's face, who retaliated with stiff jabs.
"I was pretty shot, but that's what I trained for. Even when it's on empty you change down a gear."
Left to the judges, the final decision could not have been closer and understandably the announcement of Calver's name sparked both outrage in the Campbell corner and unrestrained joy from the home-town crowd.
A rematch would be a natural to clear up any lingering doubts, and certainly would be welcomed by fight fans who had already got their money's worth.
Still, both men deserve a little breathing room to recover before those talks begin.