LAS VEGAS -- UFC featherweight champion Conor McGregor has never backed down from a challenge.
Not even the prospect of going up two weight classes made McGregor flinch.
But for all his valiance, moving up 25 pounds turned out to be a bridge too far.
Nate Diaz, who took a UFC 196 main-event slot against McGregor on two weeks' notice, submitted McGregor in the second round of their welterweight main event at the MGM Grand Garden Arena. The time of the tapout was 4:12.
"I knew it was going to be a war," said Diaz (19-11). "If I'm not in the best shape, I gotta start slow, save my energy, and then pick it up."
It was a bout that was as wild as the brief-but-crazy buildup, which featured frequent profane outbursts and a skirmish between the duo at Friday's weigh-ins.
McGregor got the better of Diaz in the opening round, landing the strongest shots and and busting open Diaz's right eye.
But a bloodied Diaz buckled McGregor with a big left to the jaw in the second. Then he found his footing, taunting the fading McGregor and landing shot after shot.
McGregor shot for a desperation takedown, but that only put him in Diaz's wheelhouse. Diaz deftly maneuvered into position and ended up with his 12th career submission victory.
The featherweight champ (19-3) made no excuses for the loss.
"I took the chance," McGregor said. "I thought I took first round, I was inefficient with my energy. I respect Nate, he took the punch. These things happen in MMA. I took a chance and it didn't work out."
The bout was initially scheduled to feaure McGregor moving up in weight to challenge lightweight champ Rafael dos Anjos, but dos Anjos pulled out two weeks before the fight with a broken foot. Diaz was picked among several candidates, including Donald Cerrone, to take his place.