Floyd Mayweather Jr has defended his rant at broadcaster Larry Merchant which prompted the 80-year-old to claim "I wish I was 50 years younger and I'd kick your ass".

Mayweather won the WBC welterweight title with a fourth-round knockout of Victor Ortiz on Saturday night.

But he grew irritable at HBO commentator Merchant's repeated questions about the controversial nature of the win, which saw Mayweather catch Ortiz with two decisive blows when the champion believed referee Joe Cortez had called break.

The 34-year-old American fighter interrupted Merchant during the in-ring post-fight interview on live television, saying: "You never give me a fair shake.

"So I'm gonna let you talk to Victor Ortiz, all right? I'm through. They can put somebody else up here to give me an interview. HBO need to fire you, you don't know s*** about boxing. You ain't s***."

Merchant then also lost his cool.

"I wish I was 50 years younger and I would kick your ass," he responded.

Mayweather later defended his comments.

"I don't regret what I said," he insisted in quotes reported by boxingscene.com.

"Everybody is tired of Larry Merchant. Everybody is tired of him. He said if he was 50 years younger, that he would kick my ass.

"All of these boxing experts...how can you be a boxing expert if you never had a fight before?"