A 21-year-old American scored a 3rd round KO with a booming right hand that left his opponent flat on his back

• A young American boxer scored a brutal knockout Wednesday on Showtime.
• Brandun Lee separated Samuel Teah's senses before his opponent had even hit the deck.
• Watch the brutal finish right here.

A 21-year-old American called Brandun Lee scored a third-round knockout with a booming right hand that left his opponent Samuel Teah flat on his back Wednesday during a ShoBox main event broadcast on Showtime.

With perfect toe-heel alignment, his left foot forward, and his elbows tucked to the side of his body, Lee was in a good position to switch from defense to attack before he threw his final punch of the fight.

Up until that moment, he had out-landed Teah when it came to jabs (10 jabs to 4), power punches (38 to 30), and total punches (48 to 34), according to Compubox data sent to Insider.

The 49th punch Lee landed was one Teah could not take, as the youngster looped his right hand over the top, shifting that body-weight onto his front foot and landing his gloved fist directly on Teah's jaw.

The impact snapped his boxing rival's neck back, and Lee appeared to walk off before Teah had even hit the deck.

From there, Teah's body stiffened, he was out cold, and there was no need for a full count.

Lee had won.

Watch the finish right here:

Teah was the toughest test of Lee's fledgling pro career, and he admitted that it was a step-up in competition for him post-fight but said he didn't learn much from the bout itself.

"I knew he couldn't outbox me," said Lee in a statement sent to Insider.

"I knew he couldn't out-punch me. I knew the knockout was going to come, and that one was definitely one of my top one or two favorites so far in my career.

"It was a step up in competition, and even the betting odds were way closer than usual," Lee said.

"I was getting a lot of DM's on Instagram and Twitter of people telling me that Sam is going to be tough. Sam is going to beat you. But hopefully, the doctors check him out, and everything is all good."

The victory advanced Lee's flawless boxing record to 22 wins (20 knockouts).

Elsewhere on the card that took place behind-closed-doors at the Mohegan Sun Casino in Uncasville, Connecticut, Jordan White beat Misael Lopez in a super featherweight match.

In a lightweight bout, Victor Padilla defeated Thomas Velasquez, and Steven Ortiz beat Jeremy Hill in a 135-pound clash.