Conor McGregor suffered a brutal KO at the hands of Dustin Poirier to leave his career on the rocks.
The Irishman was returning to the cage after a 12-month absence as he looked to mount another lightweight title challenge.
But instead he was badly beaten as Poirier dropped the former two-weight ruler in the second round before finishing the fight on the ground.
"It's hard to overcome inactivity," said McGregor. "Dustin is some fighter. I have to dust it off and come back and that’s what I’ll do.
"I'll take my licks. I’m gutted... I put the work in. I’m proud of my work."
McGregor and Poirier were meeting six years after the Irishman had KO'd his rival inside two minutes.
He went on to become the featherweight and lightweight champion while Poirier fell short of being crowned 155lb champion when he was choked out by Khabib Nurmagomedov two years ago.
The Louisiana native brought the more consistent and impressive record to the cage on Fight Island with McGregor enjoying just 40 seconds of action since his defeat by Nurmagomedov in 2018.
And it was clear from the opening round that McGregor faced a much stiffer test against Poirier with the former interim champion taking down the favourite.
And Poirier increased the pressure in the second round, targeting McGregor's calves to leave the former champion hobbling on his lead leg.
And when Poirier launched a frenzied attack, McGregor attempted to fight fire with fire but was dumped on the canvas before referee Herb Dean stepped in.
Earlier on the main card, UFC debutant Michael Chandler made a huge statement on his UFC debut with a first-round stoppage of Dan Hooker.
Chandler rocked the New Zealand lightweight with his right hand, dropped him with his left and finished the job on the ground.
He said: "It's been such a long road but every road has led me to this.
"Let's go get that title shot soon. This is the greatest moment in my professional life; Conor McGregor, there's a new king in the lightweight division.
"Khabib, if you come back for No. 30, you have to beat somebody. So try and beat me if you can."
Scotland's Joanne Calderwood took a step towards a flyweight title shot with a dominant points win over Jessica Eye.
Also on the card, Floyd Mayweather-managed Makhmud Muradov dominated Andrew Sanchez before a stunning right hand set up the finish in the third round.
Rising star Amanda Ribas saw her winning run come to a shuddering halt as she suffered a second-round stoppage defeat by Marina Rodriguez.