LAS VEGAS – Fighting a late replacement and at a weight six pounds higher than he anticipated didn’t impact Kenneth Sims Jr.’s performance Saturday.

Chicago’s Sims broke down Rock Myrthil and stopped him in the fifth round of their eight-round welterweight fight on the Frank Martin-Michel Rivera undercard inside The Chelsea at The Cosmopolitan of Las Vegas. Sims bombarded a tired Myrthil with power punches, which caused referee Celestino Ruiz to step between them and halt the action at 1:12 of the fifth round.

Sims, a junior welterweight contender, improved to 19-2-1 and recorded his seventh knockout.

The Haitian-born, Los Angeles-based Myrthil (17-2, 13 KOs), who took this fight on four days’ notice, was stopped in a second straight bout. Kevin Johnson (12-2, 8 KOs), who is scheduled to fight on this card later Saturday night, defeated him by ninth-round technical knockout in his previous fight, which took place in November 2021 at Park MGM’s Dolby Live.

Sims sensed at the start of the fifth round that a fatigued Myrthil was susceptible to getting knocked out. He wasted little time unloaded hard rights and lefts, which prompted

Myrthil seemed fatigued by the start of the fourth round. Sims cracked him with back-to-back right hands that buzzed Myrthil in the opening minute of that round.

Sims knocked Myrthil into the ropes with another right hand later in the fourth round. Sims then bombarded Myrthil with power punches, which seemed to stagger Myrthil as he backed into his own corner.

Sims countered Myrthil with a right hand just in the first minute of the third round. A counter right by Sims knocked Myrthil off balance later in the third round.

By then, Sims’ switching from an orthodox stance to a southpaw stance neutralized Myrthil, who had a strong first round.

Sims switched to a southpaw stance several times in the second round. He landed a hard right hook in one of those instances during the final minute of the second round.

Sims started the second round working well off his jab.

Myrthil landed the harder punches with more regularity in the opening round. Myrthil drilled Sims with a right hand in the final minute of the first round, but Sims took that flush punch well.

In the bout before Sims’ victory, Ray Ray Robinson defeated Victor Saravia by unanimous decision in Robinson’s four-round pro debut.

The left-handed Robinson, a southpaw from Cincinnati, won by the same score on all three cards, 40-35. Saravia (1-4-1, 1 KO), of North Hollywood, California, is 0-4-1 since he won his pro debut by knockout.

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