Leeds fighter Josh Warrington was stunned on home soil on Saturday as Luis Alberto Lopez won on points to take the IBF featherweight world title.
Warrington, 32, was hoping to stage a successful defence of the title he reclaimed in March.
But Mexican Lopez, 29, edged a close fight to hand Warrington the second defeat of his professional career.
The decision was greeted by boos from the disappointed home fans at Leeds Arena.
Warrington’s future is now uncertain, with victory expected to have led to lucrative fights abroad.
“I understand the fight was close in stages,” Warrington told BBC Radio 5 Live. “He started off like a train and kept coming forward.
“I kept working inside and could hear him wincing to the body constantly. He made a meal of it when I hit his leg and I was surprised Bob Williams let him have 20 seconds.
“In hindsight I look back and think I could have done more.”
Warrington unable to deliver for expectant home crowd
This was the first time Warrington had taken to the ring since beating Spain’s Kiko Martinez with a seventh-round stoppage nine months ago to win back the belt he had first held between 2018 and January 2021.
Warrington won that contest despite sustaining a broken jaw during the bout and the Englishman had been out of the ring since to recuperate.
His return to the Leeds Arena for a 12th time on Saturday drew a huge and vocal home crowd, who were hoping to see their hero triumph having earlier watched England lose to France in the World Cup quarter-finals. The boxing had been delayed until the outcome of the football.
The atmosphere was electric as Warrington emerged shortly before 23:00 GMT, but straight from the outset Lopez made life difficult for him with his movement and aggression.
It was not a good sign for Warrington after his nose was bloodied in the opening round, while Lopez continued to use his speed to evade Warrington’s powerful throws.
The Brit was never able to settle into a comfortable rhythm but there was little between the two over the next few rounds with the fight looking capable of going either way.
Lopez grew in confidence as the rounds ticked away but a strong ninth for Warrington looked like it might be a turning point as he landed some strong shots.
Warrington then unloaded everything he had in the final couple of rounds, hurting Lopez in the eleventh and final round but ultimately it was not enough.
“I’m really happy at the moment,” Lopez said after the upset victory.
“It has been a long period of preparation, this is my moment. I’m delighted to be world champion.”
What next for Warrington?
A victory would have opened several doors for Warrington.
There was the possibility of an all-British featherweight title fight with WBA ‘Regular’ champion Leigh Wood or a possible debut in the United States, with Warrington having long-been keen to given his fans the chance to watch him fight overseas.
Now Warrington is at a crossroads after losing to a fighter who had two defeats on his record and was untested at an elite level.
Retirement is now a stronger possibility, but Warrington was non-committal about his future after the fight.
“I had aspirations of doing other things,” he said.
“I wanted to go to the States and I still want to do that. Maybe unifying is a step too far but I think I can still fight one of the other champions.”
The reigning champions at featherweight are WBA (Super) belt holder Leo Santa Cruz, WBC belt holder Rey Vargas and WBO champion Emanuel Navarrete – any of those men would be tough fights for Warrington.
Bridges defends title in all-Australian grudge match
In the chief support fight of the night, Ebanie Bridges retained her IBF bantamweight world title with an eighth-round stoppage of Shannon O’Connell in a feisty all-Australian clash.
O’Connell had been fully confident of beating her opponent in a build up that saw plenty of words exchanged between the pair and she landed a big right hook in an aggressive first round to let Bridges know she meant business.
But Bridges battled back, dropping O’Connell with a bruising punch to the chin in the third round and, after a couple of more even rounds, the defending champion took control in the seventh with a flurry of hits causing her opponent’s left eye to swell.
Bridges then sealed the victory in the following round with a barrage of attacks that led to referee Howard Foster stepping in.
“I’m not usually disrespectful but she’s been talking trash all week,” said Bridges afterwards. “This is boxing, we’re going to get hit hard and I hit her with 10 back.
“I hope Australia is proud of me and you know who the Queen of Australian boxing is now.”
Highlights of Josh Warrington’s fight with Luis Alberto Lopez will be available on the BBC Sport website and app from 09:00 GMT on Sunday, 11 December.