BROOKLYN - Undefeated rising star Errol "The Truth" Spence Jr. (21-0-, 18 KOs) moved one step closer to a world title as he delivered a thunderous sixth-round knockout of tough Italian contender Leonard "The Lion" Bundu (33-2-2, 12 KOs) Sunday afternoon in the main event of Premier Boxing Champions (PBC) on NBC from Ford Amphitheater at Coney Island Boardwalk in Brooklyn.

With the knockout, Spence Jr. becomes the mandatory challenger for undefeated welterweight Kell Brook and showed his willingness to step in the ring with Brook.

In a prime spot that immediately followed the U.S. Men's basketball team winning Olympic gold in Rio, the Dallas-native started patiently, measuring his opponent with steady jabs. Bundu came ready to fight and tried to make it difficult for Spence Jr. by switching stances and attacking from awkward angles.

Spence Jr. began to pick up his power punching as the rounds went on and started to connect with left uppercuts and hooks. After round four, Bundu returned to his corner with swelling beginning to show up over his right eye.

The dominance with his power punches began to wear Bundu down as the fight entered the sixth. After seemingly scoring a knockdown, which was ruled a slip by referee Johnny Callas, Spence Jr. quickly pounced on Bundu and trapped him in the corner.

"He certainly lived up to the hype," said Bundu, who was taken to the hospital for evaluation after the fight. "Errol was more precise than Keith Thurman. Errol threw more punches and was fresh in every round."

It didn't take long for Spence Jr. to take advantage as he hit Bundu with a hard left uppercut and followed with a crushing right hook that put Bundu on the mat for good and forced the referee to stop the fight at 2:06 of the sixth-round.

"I really tried to get up but I couldn't," said Bundu, who had never been stopped before. "I am okay though. I feel good."

Spence Jr. won every round on all three judges' cards and landed a staggering 64 percent of his power punches, per Compubox.