LAS VEGAS – Wilder hit the canvas hard Saturday.

No, not that one.

Marsellos Wilder, the unbeaten WBC heavyweight champion’s younger brother, suffered the second and most definitive knockout defeat of his brief career on the Deontay Wilder-Luis Ortiz undercard at MGM Grand Garden Arena. The younger Wilder was in control of their scheduled six-round cruiserweight contest, until Dustin Long landed a left hand that knocked Wilder out cold in the fourth round.

A loud thud was heard throughout the arena when Wilder hit the canvas. Referee Jay Nady immediately waved an end to the fight 1:51 into the fourth round.

The 37-year-old Long (3-1-2, 3 KOs), of Johnson City, Tennessee, won a third straight fight.

The 30-year-old Wilder (5-2, 2 KOs, 1 NC), of Northport, Alabama, had won each of his three previous bouts, each by unanimous decision, following a surprising, fourth-round technical knockout loss to journeyman William Deets (8-13, 3 KOs) on January 26 at Barclays Center in Brooklyn. The first of those three wins was changed to a no-contest.

In his prior bout, Wilder defeated Nicoy Clarke (2-5) by unanimous decision in a four-rounder August 3 at Barclays Center.

Wilder previously out-pointed Tyler Vogel (3-3, 2 KOs) by unanimous decision in a four-rounder June 29 at NRG Arena in Houston. The result of his first fight after Deets defeated him, a first-round knockout of Mark Sanchez (0-2, 1 NC) on March 16 at AT&T Stadium in Arlington, Texas, was changed to a no-contest in April by the Texas Combative Sports Program because Wilder failed an unspecified test it administered.

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.