Twenty-six seconds: Francis Ngannou finishes Cain Velasquez in half a minute at UFC Phoenix

The main event at UFC Fight Night in Phoenix, Arizona produced a quickfire finish as the combination of a Francis Ngannou uppercut and a collapsed knee saw Cain Velasquez stopped in just 26 seconds of the night's main event bout.

The action had barely started at the Talking Stick Arena in Phoenix as former title challenger Ngannou and former two-time heavyweight champion Velasquez kicked off their main event bout.

And when the pair locked up close to the fence, a right hand from Ngannou stunned and dropped Velasquez, whose right knee collapsed as he fell to the canvas in obvious pain. 

It brought the highly-anticipated bout to an abrupt end as the fans booed their disappointment at the event's conclusion.

"I promise you guys, I'm back," said Ngannou after the fight.

"And you're going to see me around again. I told you yesterday I had a surprise for you. I hope you like it."

Ngannou admitted he wasn't entirely sure what had transpired until he saw the replays on the big screen

"He was very fast, so I didn't even realise what happened," he admitted.

"I've been waiting for this fight since two years ago because I knew I had to face the adversity of a fighter like Cain to prove myself. I'm honored, and I'm also happy to have the win.

Velasquez, meanwhile, begged the crowd not to boo Ngannou after the fight, and admitted he made a tactical error that led to his defeat.

"I stood a little too close, too early," he explained.

"Yeah [my knee] just kinda buckled there with the shots. I stood a little too close, a little too early. That's where he's dangerous. That's how it happened."

And the clearly crestfallen former Arizona State wrestler said he would bounce back once again after his latest injury setback and return to the octagon again.

"As soon as I'm all good, you know?" he said.

"Man, I felt super-ready for this fight. I came in here super calm and relaxed, and ready to go.

"This is why this sport is so great - you never know what's going to happen.

"I'm just truly sorry for this."

Immediately after the fight there was some debate over whether an Ngannou punch had landed during the final flurry, but further replay showed that the Cameroonian had indeed connected with a big uppercut on the former champion.

There were plenty of post-fight reaction online as Tweeters offered their takes on the blink-and-you'll-miss-it finish to the main event.

American reporter Jose Youngs called Ngannou's punch power "the touch of death," while Canadian analyst Robin Black confirmed the finish was due to Ngannou's uppercut, saying it was "clean" and "brutal".

Former UFC fighter and analyst Kenny Florian agreed, saying the punch "knocked Cain off balance and caused him to tweak his knee".

But that assessment was refuted by Velasquez himself, who said the fight was finished as a result of his knee collapsing, rather than any strikes landed by his opponent.

"It was just the knee, yeah," insisted the former champion during the post-fight press conference.

"He didn't even touch me."