Jay Harris fell short in his bid to become WBC world flyweight champion despite producing a career-enhancing display against Julio Cesar Martinez.
The Welshman lost a unanimous points decision against the relentless Mexican but gave a gutsy performance.
Harris, 29, was knocked down in the 10th round but finished the fight as the aggressor in Frisco, Texas.
The judges scored the contest 118-109, 116-111 and 115-112 in favour of the champion.
"Obviously I am very, very disappointed, but I can take a lot from this," said Harris.
"I think 115-112 was about right, maybe I thought I nicked a draw, but as everyone saw, I gave it 100%.
"I was getting booed going out and it sounded like I left the ring with a stadium full of fans. I showed I am a world-level fighter."
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Harris, who was booed into the arena, was given a standing ovation on the way out after his first defeat in his 18th professional contest.
He was always a huge underdog against Martinez, who promoter Eddie Hearn believes can be "a generational fighter".
The first round bore out that prediction as Martinez started like a wrecking ball, overwhelming Harris with fast combinations with the Welshman failing to put distance between them and utilise his superior reach.
Harris - who works two nights a week loading boxes at an Amazon warehouse - was always an underdog against the hard-hitting Mexican, but he never looked overwhelmed fighting in front of thousands at the Ford Center.
Indeed, as soon as the second and third rounds Harris had managed to largely silence the partisan crowd, using his reach and size advantage to quell the onslaught of Martinez shots.
Harris has won EBU, IBF Inter-Continental and Commonwealth titles and has notable names on his record such as Paddy Barnes, but this was a completely different level for him.
Martinez, undefeated since a debut defeat, has been building a head of steam in the flyweight division. He powered through the challenge of Wales' prodigious talent Andrew Selby in Mexico City last year and also dominated Charlie Edwards at the O2 Arena in a fight that ended as a no-contest because of a low blow.
Martinez' relentless front-foot style - he seemed to constantly hold the centre of the ring - meant he threw and landed more shots, but Harris did trouble him and was always dangerous.
The tide looked to turn decisively against the Townhill boxer in the 10th round when two booming body shots - a left to set up a right - floored Harris, who took an eight-count, but he showed fantastic resilience to battle back and survive the round.
That took the wind out of Martinez and Harris finished the fight the aggressor, with the 24-year old champion seemingly relieved when it was all over.
In his post-fight interview, Martinez said via translator that Harris was his toughest opponent to date.