Cork middleweight Gary "Spike" O'Sullivan was retired by his corner versus Mexico's Jaime Munguia in the early hours of this morning in Texas.
Coach Paschal Collins threw in the towel 2 minutes and 17 seconds into the 11th round with the Corkman soaking up punishment at the Alamodome in San Antonio.
The win improves unbeaten Munguia, who was making his middleweight debut after moving up from light-middle to 35 straight victories and enhances his KO ratio to 80%.
O'Sullivan, who battled bravely throughout, drops to 30 wins from 34 outings and his fourth defeat to a world champion.
"A fighter always wants to fight on, but sometimes you're too brave for your own good," said O'Sullivan who admitted he was gutted to lose.
Munguia was docked a point for a low blow in the 6th round and O'Sullivan took a knee in the 7th after another punch flew south of the border, but the Tijuana puncher escaped further sanction from Texas ref Mark Calo-oy
Munguia won the first few rounds, but O'Sullivan rattled the former WBO World champion in the second and buzzed his opponent in the fifth with headshots.
Munguia's coach Erik Morales, a multiple world champion who beat Ireland's Wayne McCullough and Manny Pacquiao in the past, told his man to go back to using his speed and left jab from the eighth round up.
Munguia redeployed those tactics to good effect going down the final stretch with O'Sullivan picking up a cut near his left eye.
O'Sullivan continued to find the target in the later rounds, but it all began to unravel in the 11th when Munguia started using his power jab as the triggering mechanism for rights hooks and the Mahon fighter was withdrawn by his corner.
"I think he is strong at middleweight and he can do good things at the weight. Golden Boy (promoters) said they are going to get me a world title shot at light middleweight," added O'Sullivan.
"I think he was stronger at 160lbs than he was at 154lbs. He was the bigger guy in there, but I nearly took him out a few times as well.
Munguia was happy with his performance at the new limit.
"I felt the difference at 160 pounds. I waited for him to get tired and then find the finish, "said the 23-year-old who is targeting fights with Saul Canelo Alvarez and Gennadiy Golovkin. "I want them all," he added.
Ring legend and Golden Boy chief Oscar De La Hoya, who promotes O'Sullivan and Munguia, hailed the Leesider's gutsy performance against an opponent who is 13 years his junior, according to Conal Thomas who was was part of O'Sullivan's team in Texas.
O'Sullivan and team are due to arrive home for the USA tomorrow.