American brawler Dustin Poirier overpowered Conor McGregor on Saturday in Las Vegas, winning their lightweight UFC trilogy fight by a TKO after the Irishman appeared to snap his left leg in the waning moments of the opening round.

The fight was officially ruled as a "doctor's stoppage" at the end of the first round after a bloodied McGregor swung and missed a punch then stumbled backwards, his lower leg grotesquely buckling under him near the edge of the octagon. 

Poirier then pounced on a helpless McGregor, raining down a series of punches and elbows until McGregor was saved by the bell in front of the capacity crowd at the T-Mobile arena. 

"This guy was saying he was going to murder me and kill me. I am going to leave here in a coffin," said Poirier. 

"Murder is something you don't clown around with. You don't talk to people like that. I hope this guy gets home safe to his beautiful family."

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Promoters said afterwards that McGregor's injury was to his lower tibia and that he was expected to undergo surgery on Sunday to repair the damage.

The scheduled five-round rubber match was expected to settle the score and complete the trilogy between the former two-weight UFC champ McGregor and his longtime rival Poirier, who now looks set to get a shot at an undisputed title and establish himself as one of the sport's superstars.

There was plenty of bad blood between the two in the lead up to their third fight. McGregor insulted Poirier's wife in a tweet and told Poirier he was going to leave the ring in the box.

Instead McGregor had to be carted out on a stretcher after doctors stabilized his leg.

"He fractured it on one of the checks at the beginning, then broke it with the punch," Poirier said. "I felt something. He was kicking me hard. It was probably cracked then on the twist and punch [it broke]."

Poirier said his payback was karma. "This guy is a dirt bag," Poirier said. "Sometimes these things happen."

In an expletive-laden interview inside the octagon as he waited for the stretcher to take him out, McGregor insisted he was winning the fight when the freak accident happened.

"I was boxing his bleeding head off. Kicking his bleeding leg off. This is not over," said McGregor, who has had just one win in four and a half years.

In 2014, McGregor knocked out Poirier in under two minutes and Poirier returned the favour in January, demolishing McGregor with a second round KO.

It was the first time a capacity crowd has attended a UFC fight in Nevada since the Covid-19 pandemic. With the United States lifting restrictions across the country, fans are returning to sporting events.

- Trump booed -

Saturday's crowd included former US President Donald Trump, who was booed by a large contingent of the MMA crowd, while others chanted "U-S-A."

The first round started with both fighters exchanging kicks and blows, with Poirier getting the better shots in, and it didn't take long for McGregor to start bleeding from the mouth.

Poirier landed a vicious left hand, a few more long kicks and then they went to the ground for about two minutes of grappling. Poirier let McGregor get up and they both threw a punch at the same time but missed, and that's when McGregor's leg buckled.

"When I was walking away and he stayed down, I looked down at his shin and I saw the bone. It was disfigured a little bit. I said, 'Oh man.'" Poirier said.

Promoter Dana White said he hoped there was a fourth Poirier-McGregor fight.

"You can't have a fight finish that way," White said.

Poirier had his own take on another fight.

"We are going to fight again whether it's in the octagon or the sidewalk," Poirier said.