By Keith Idec

LAS VEGAS – Gervonta Davis’ supposedly easy fight against Francisco Fonseca went about as smoothly as their weigh-in.

A day after he was stripped of the IBF super featherweight championship because he was overweight, Davis knocked out Fonseca in controversial fashion on the Floyd Mayweather Jr.-Conor McGregor undercard. Baltimore’s Davis was winning their fight through seven rounds, but he hit Fonseca with an illegal punch behind his head and the huge underdog couldn’t continue at T-Mobile Arena.

Fonseca went down from Davis’ left-handed rabbit punch, and stayed on his hands and knees until referee Russell Mora counted him out at 39 seconds of the eighth round. Davis (19-0, 18 KOs) said he hurt Fonseca (19-1-1, 13 KOs) with a left hand to the body just before landing the rabbit punch and questioned how hurt Fonseca actually was from that illegal blow.

“I actually caught him with a body shot before that and he was hurt,” Davis told Showtime’s Jim Gray in the ring after his win. “So he took advantage of me hitting him in the back of his head and went down.”

Costa Rica’s Fonseca disputed Davis’ version of what happened.

“It was an illegal blow,” Fonseca said through a translator. “Everybody saw that when I was going down he hit me twice with an illegal blow. I just want a rematch.”

After landing the rabbit punch, Davis also shoved Fonseca to the canvas with another left hand behind his head as Fonseca was falling from the first illegal blow.

The controversial conclusion to their fight drew boos from the crowd and ended a difficult two-day stretch for the undefeated former champion. The 22-year-old Davis was the youngest American champion in boxing before he was stripped Friday, when he missed weight at 132 pounds.

Fonseca, 23, weighed in at 130 pounds, thus the IBF title was at stake for him Saturday night.

Costa Rica’s Fonseca hadn’t fought anyone of note before Saturday night and mostly had boxed in his home country since he turned pro in December 2013. But he came to fight, despite that he was listed as a 35-1 underdog at MGM Grand’s sports book.

He had some moderate success against Davis in the first two rounds, until the powerful southpaw started unloading on him midway through the third round. Davis began toying with Fonseca in the fourth round, when he put his hands behind his back and mocked Fonseca’s power by pretending he was hurt after Fonseca landed a punch.

Fonseca got rough with Davis during the seventh round by leading with his head and pushing Davis into the ropes. Referee Russell Mora warned Fonseca for his questionable tactics in that round.

Fonseca also drilled Davis with a straight right hand in the seventh round, as Davis was off-balance, against the ropes.

Davis recorded his 10th straight knockout victory Saturday night. He didn’t look as sharp, however, as he did during his first two fights in 2017.

Davis won the IBF super featherweight title by stopping Puerto Rico’s Jose Pedraza (22-1, 12 KOs) in the seventh round January 12 at Brooklyn’s Barclays Center. He defended the title once, when he beat England’s Liam Walsh (21-1, 14 KOs), his mandatory challenger, by third-round technical knockout May 20 in London.

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