Scott Gammer says he will regain the British heavyweight title and wants a rematch with conqueror Danny Williams.
"I don't know what went wrong, I know my fitness was good but I just felt drained from the first round," he said after his KO by Williams in the ninth.
"I'll be back in the gym within a week, then I'll fight as soon as possible.
"It was my first defeat and a big one, but I'll be back stronger. I'm going to get the belt back and there's no shadow of a doubt that I want a rematch."
The 30-year-old Pembroke Dock man's prospects of an immediate return with Williams seem slim, the Londoner likely to target bigger fights if he does not follow through on a possible plan to retire.
GAMMER v WILLIAMS FEATURE
But 33-year-old Williams could vacate the belt in either event, opening up the chance of another title chance for Gammer, whose record now stands at 17-1-1 (9 KOs).
Williams, stopped in three rounds by Harrison in an awful performance in December, weighed in at a lightweight 16st 4lb, the lightest he had been since his professional debut in 1995.
After the fight, he claimed a set of his own scales led him to believe he was more than 19st, but in any case it was clear that Gammer's decision to come in at a career-high 17st 1lb cost him expected advantages in speed and work-rate.
"Scott boxed a lot better than I anticipated," said Williams, whose record moves to 37-6 (30 KOs) after the win at the Cwrt Herbert Sports Centre in Neath.
"Looking at Danny and the way he fluctuates in form I thought the time was right to take him"
"I thought that he was a come-forward fighter with no boxing skills, but he threw good combinations.
"In the eighth I could see him getting excited at the shots he was throwing, but I knew I would get him in the next round because of the effort he was putting in without hurting me.
"The local crowd was encouraging him and making him throw more, but I just covered up and showed my experience.
"He has to improve on what he does and come back - I'm sure he can win the British title again."
Gammer's promoter Paul Boyce admitted that he had miscalculated in taking the Danny Williams fight, having turned down a bigger-money match with Audley Harrison in London.
"I got it wrong and I take responsibility," said Boyce.
"Looking at Danny and the way he fluctuates in form I thought the time was right to take him.
"But he just looked so good, he stunned us all."