UFC star Jorge Masvidal set a UFC record Saturday -- knocking out Ben Askren in 5 seconds!!!

The two faced off at UFC 239 in Las Vegas -- after months of some serious trash talk between the welterweight stars. 

When the opening bell rang, Ben -- a highly decorated wrestler -- put his head down and went for a takedown ... something Masvidal clearly expected. 

Masvidal had clearly been training for that move -- and came sprinting out his corner with a flying knee that connected right to Askren's temple. 

Askren was out cold before he hit the mat -- but Masvidal continued to smash him until the ref stepped in and stopped the fight. 

Askren eventually woke up -- he was helped to the back where he was treated by medical staffers. He was later transported to a local hospital. Hopefully, he's okay. 

As for Masvidal ... wow.