Matthew Macklin was knocked out by Jorge Sebastian Heiland in the 10th round of their WBC world title eliminator in Dublin.
The Birmingham-based Irishman trailed on points when he was floored by a right hook from his Argentine foe.
"I'd been tiring for the three or four rounds before that," said Macklin, 32.
Meanwhile, IBF, WBA and WBO heavyweight champion Wladimir Klitschko completed a 17th consecutive title defence as he knocked out Kubrat Pulev in round five.
The 38-year-old Ukrainian floored his Bulgarian opponent twice in the opening round and again in the third.
The stoppage came in round five as Klitschko extended his record to 63-3 with the 53rd knockout of his career.
Defeat for Pulev was his first in 21 fights.
Heiland, meanwhile, will now get a shot at the WBC world title, held by Puerto Rican Miguel Cotto.
"I can't take anything away from Heiland, he fought a good fight and upped it at the right time," he said.
"I'll have to look at the performance and see where we go from here. I'm not going to make hasty decisions, I feel that at my best, I'm still world class."