Kubrat Pulev startled Frank Mir with a big right hand before the former UFC champion was rescued by the referee in the first round of their Triad Combat Rules fight.
The Bulgarian heavyweight ruthlessly stopped Mir, who wobbled dramatically into the ropes after Pulev followed a crunching right with a cuffing left hook, and the bout was mercifully waved off.
Pulev was returning to action for the first time since he was stopped in the ninth round by Anthony Joshua in their world heavyweight title fight last December.
Under the rules of the Triad Combat bout, both fighters wore MMA gloves and were allowed to hit and hold in a triangular ring.
But Pulev's superior punching was quickly on show as a single straight right left Mir unable to continue in the closing seconds of the opening round.
Pulev hopes to resume his boxing career next year, with promoter Ivaylo Gotzev telling Sky Sports that he had been contacted about a fight against British contender Joe Joyce.
"We've been getting a lot of offers. Joe Joyce is looking for us," Gotzev told Sky Sports.
"Kubrat is definitely not a stepping stone for no-one, so we're going to pick and choose our battles, and get him back to where he feels great.
"It's all about how he wants to enter the ring and how long he wants to keep going for."