Conor McGregor sustained a serious leg injury as he lost to lightweight rival Dustin Poirier via doctor's stoppage in round one at UFC 264 in Las Vegas.
The Irishman, 32, started aggressively by targeting Poirier with leg kicks, but the American quickly capitalised after seeming to hear McGregor’s leg crack during one exchange.
Poirier, also 32, took his opponent to the ground, where he dominated before the fighters returned to their feet.
Both swung and missed and McGregor stepped back before his leg collapsed beneath him. McGregor somehow survived until the end of the round when the bout was stopped.
After the fight, UFC president Dana White said McGregor had broken "the lower tibia in his left shin".
In a dramatic postscript to the encounter, their third following McGregor’s victory in 2014 and his defeat in January, Poirier called his opponent a "dirtbag" when interviewed by commentator Joe Rogan afterwards.
Poirier accused McGregor of pushing his fingers inside his gloves as they were grappling and said he was trying to tell referee Herb Dean about the cheating.
McGregor made his own claims to Rogan.
"I was boxing the ears off him and kicking the legs off him," McGregor said. "This is not over."
UFC chief White said that he expected McGregor to need surgery but that he would then fight Poirier for a fourth time.
"It sucks, it's brutal, it's not the way you want to see fights end. Dustin Poirier will fight for the title and when Conor is healed and ready to go, you do the rematch, I guess," he said.
"The fight didn't get finished. You can't have a fight finish that way. We'll see how the whole thing plays out. I mean, who knows how long Conor is out? Poirier will do his thing until Conor is ready."
Former United States president Donald Trump was among the 20,062 fans in a sold-out T-Mobile Arena that also heard McGregor launch an expletive-ridden post-fight tirade at Poirier, insulting the American's wife in the process and sarcastically inviting her to an after-party.
"I don't like that. That's not good. Leave people's families and wives and all that stuff out of it. Family has nothing to do with it," White said.
The loss takes McGregor to 1-3 in the cage since 2016, when he left the UFC to take on Floyd Mayweather in boxing.
With the victory, Poirier has earned the chance to recapture the lightweight title he held on an interim basis in 2019 prior to a loss to Khabib Nurmagomedov, who retired last year with a perfect 29-0 record.