The left-handed American sent Ashley to the canvas twice in the fifth round and then put him down three more times in the sixth, all with left- handed body blows. The three knockdown rule was then put into effect, which requires a fight to be stopped if there are three knockdowns in one round.
'You see when I'm aggressive,' Nunn said, 'I'm a bulldog. I showed that.' The doctor at ringside, Jacinto Hernandez, said Ashley might have suffered a broken rib in his right side.
Ashley, whose 18 victories have all come by knock-out, gave Nunn some problems in the first two rounds, but Nunn rocked him several times in the third round and ended the fourth with a vicious left to the body, after hurting him several times with lefts to the head.
Ashley, who had moved down from the light-heavyweight division for the bout, said: 'I think I came down too far (in weight). He's too much of a puncher. Those body shots were just killing me. I was too weak.'
Ashley appeared to hurt Nunn with a straight right early in the sixth, but Nunn insisted he had not been hurt.
'He's a tough guy,' Nunn added, 'but he's fighting the world champion.' About 5,000 fans watched the bout in an arena that seats 22,000 for basketball.
With this victory, Nunn, the former International Boxing Federation middleweight champion, has a 10-1 record in world-title bouts. It was his third defence of the title he won with a 12-round points decision over Victor Cordoba last September.
After the victory, Nunn issued a challenge to the IBF super-middleweight champion, James Toney, who stopped him in the 11th round when they were middleweights in 1991.Reuse content