The 34-year-old former 2x heavyweight champion was sharp from the very start of the bout ... putting 37-year-old Francis down in the first round with a hard straight right. Ngannou returned to his feet and even landed a hard punch towards the end of the round.

In the second, AJ again knocked Francis down ... but the former UFC champ beat the count, and the round continued.

But, just a short time later, Anthony uncorked a devastating right that landed flush to Francis' face, bending him backward (similar to the way Ngannou stopped Stipe Miocic), and immediately prompting the referee to stop the fight.

No count needed.

Ngannou, as doctors and medical professionals rushed in, remained down on the canvas for a couple of minutes ... and was even given oxygen.

Thankfully, Francis eventually got back on his feet, and appeared to be okay.

Despite less than 6 minutes of fighting, AJ encouraged Francis, who pushed Tyson Fury to the brink last year, to stick with boxing.

Promoter Eddie Hearn, who has worked with many of the biggest and best in the sport, called the KO punch one of the "most destructive knockouts" he'd ever seen.