Liam Smith stopped Liam Williams on a gruesome cut eyelid in the ninth round just as he looked to be heading to defeat on Saturday.
Smith (25-1-1, 14 KOs) had just enjoyed his best round of an absorbing bout that British rival Williams had the better of when the Welshman's trainer Gary Lockett pulled him out.
The fight became a bloodbath in the ninth round when after a toe-to-toe exchange Welshman Williams was cut by an accidental clash of heads.
There will be calls for a rematch since Williams (16-1-1, 11 KOs) was ahead by one point on all three scorecards, which seemed generous to Smith who was also badly cut.
"I started well and I think Liam won next three and the cut bothered me, he got momentum and then I adjusted to what he was doing," Smith said.
"I knew I was getting to him and it was a good decision by Gary Lockett because I would have stopped him the next round.
"I would have stopped him 100 per cent regardless [of the judges]."
Smith, who failed to make the weight so did not lift the WBO Interim super-welterweight title with this win, says he intends to stay in the super-welterweight division.
And Williams hopes there will be an immediate rematch after losing out on a cut.
"I did start to tire a little to be honest but in the ninth I felt OK again," said Williams.
"I want to do it again, it will never be an easy fight because he's a class fighter."
Williams, 24, went into his biggest fight yet with one defence of the British title behind him but Smith -- four years older -- had experience at the top level, was a former domestic champion and began the betting favourite.
Smith lost the WBO world title by ninth round knockout to Mexican Canelo Alvarez in September and had been hoping to put himself in contention to win the belt back.
But the Liverpool boxer weighed in almost two pounds over the super-welterweight limit and so forfeited the chance to win the vacant WBO Interim title, which was still up for grabs for Williams.
To make it more sickening for Smith, he claimed he had been told he only had one hour to lose the extra weight and so declined the chance, only to be later told he was actually allowed two hours. By then it was too late and the chance to win the interim title had gone.
Williams made a confident start on the front foot, flicking out his jab, which kept Smith out of range and trailing by the end of three rounds.
Smith also suffered a cut by his right eye from a straight left in the third round, which was an impressive session from Williams who was too sharp early on for the Englishman.
Smith -- one of four boxing brothers [along with Paul, Stephen and Callum] -- was sluggish and static, an easy target for Williams, in the fourth round and at the halfway point he had a lot of work to do if he wanted to claw his way back into the scrap.
But the cut seemed to worsen in the seventh round and with his face smeared in blood, there was a moment of concern for Smith when a doctor deliberated on whether he could continue or not.
Sensing he was behind on points, Smith began to unload heavy shots in a thrilling ninth round. Both had good moments, but Smith had the better of it and Williams ended the round with a horrible slit across his right eyelid, caused by an accidental clash of heads.
Lockett took immediate action and pulled Williams out of the fight.
"It was only going to get worse," said Lockett.