McGregor knocked out by Poirier in stunning UFC upset

American underdog Dustin Poirier shook up the world of mixed martial arts Sunday, beating up the legs of Irish superstar Conor McGregor before knocking him out at UFC 257 in Abu Dhabi.

"I'm happy but I'm not surprised. I put in the work," Poirier after the referee stepped in after 2min 32sec of the second round to save McGregor from further damage after he was dropped by a fierce combination of punches.

The 32-year-old Poirier was ranked second in the flyweight division going into the fight, but still rated a heavy underdog by bookmakers to beat his fourth-ranked opponent, arguably the most famous fighter the sport has seen.

The controversial McGregor, known as "Notorious", was returning to the octagon for the first time in a year and hoping for a repeat of the pair's first contest in 2014 which he won by TKO after just 109 seconds.

But Poirier, who now has a UFC record of 27-6, used low calf kicks in an effort to put McGregor (22-5) off his usual ferocious stand-up game -- and it worked.

"First off I want to say Conor took this result professionally. We're 1-1 and maybe we're going to have to do it again," said Poirier, hinting at the prospect of a trilogy fight.

"I'm happy with the place I am in. I'm happy with the face I see in the mirror.

"The goal was to be technical and to pick my shots, not brawl at all. I had him hurt and I went a little crazy."

McGregor likes to call himself "Mystic Mac" and had spent the week leading into the fight predicting he would end it inside 60 seconds.

But it was a different Poirier to the one who had looked overmatched when the pair previously met at UFC 178 at the lighter featherweight.

Poirier worked the legs and took McGregor down into a grapple for much of the first round -- avoiding a boxing face-off against a man who once shared the ring with Floyd Mayweather and had touted a future fight against Manny Pacquiao.

Poirier then found his range in the second, weakening McGregor's base with more calf kicks before exploding for the shock finish.

He staggered McGregor with a big overhand left and then dropped him to the canvas with a straight right flush on the Irishman's nose before referee Herb Dean waved it off.

"The low calf kick was very good but Dustin is some fighter," said McGregor.

"I'll dust it off and come back because that's what I'll do. I'll take my licks.

"I'm gutted, it's a tough one to swallow. I just want to go back to the hotel and chill with my kids and regroup."

The second-ranked Poirier is now in prime position to either inherit or fight for the UFC's lightweight belt, most recently held by the now-retired unbeaten Russian Khabib Nurmagomedov.

"I felt like this was the title fight. If Khabib's not coming back then I'm the champion," said Poirier.

The fight was held inside the new Etihad Arena on the "Fight Island" bio-secure sporting hub of Yas Island that the Abu Dhabi government established last July.

The three UFC events staged over the past week have been the first to allow fans to sit cageside, with around 2,000 in attendance for the main event.