Liam Williams was denied a world title after being dropped and frustrated by Demetrius Andrade on Saturday night in Florida.
Andrade remained undefeated and won by unanimous decision to cling onto his WBO middleweight championship despite a spirited effort from Williams.
The Brit was regarded as the champion's sternest test but got off to a catastrophic start.
Andrade hurt him in the opening round then, in the second, knocked him down with a right-left combination.
Williams gamely battled back and forced Andrade into a brawl at times, which the American regularly spoiled by holding.
The ninth round was Williams' best - a right hand rocked Andrade but he couldn't follow up with another blow and the moment slipped away.
The fight was scored 118-109, 118-109 and 116-111 in Andrade's favour.
Joe Calzaghe, who Williams was aiming to emulate as a world champion from Wales, said: "Brave effort by Williams tonight. Hurt early but came back and showed he belongs at world level hurting Andrade in the ninth.
"Bad ref, frustrating and allowed way too much holding!"
Williams, after two losses to Liam Smith in 2017, had reinvented himself and was on a superb seven-fight win streak which earned him this mandatory opportunity.
Instead, the night belonged to the southpaw Andrade who delivered a reminder that he is a tricky customer in his fourth world title defence.
Unbeaten in 30, he will be looking towards the middleweight division's other champions - Gennadiy Golovkin, Ryota Murata and Jermall Charlo.