MMA Robbery Hero -- 18-SECOND VICTORY ... In 1st Post-Robbery Fight!

The gas station robbery fight was NO FLUKE -- Mayura Dissanyake has serious MMA skills and he proved it in the ring last night by destroying his opponent in less than 20 SECONDS!!!

Of course, Dissanyake shot to fame last month when he beat the living crap out of 3 men who were trying to rob his co-worker at a gas station in Houston. Even UFC boss Dana White said he was impressed with Mayura's heroic actions.

Well, last night in Houston ... Mayura got in the ring for the first time since the incident -- and dominated his opponent Jamie Garcia in an amateur welterweight bout at Bayou City Events Center in Houston.

18 seconds. That's all it took.

"Each time I was punching, I was thinking about kicking him (while he was down)," Mayura tells TMZ Sports ... "but I had to stop myself. This isn’t the parking lot! It’s not a street fight!”

The 24-year-old -- who still works at the gas station -- says he's been in talks to train with Blackhouse MMA (Anderson Silva's gym) in California ... but so far no contract has been signed.