Aggressive puncher Ortiz had knocked down 12 of his previous 13 opponents and he pinned fellow American Mayweather against the ropes before being penalised by referee Joe Cortez for a head-butt.
After a brief time-out, Ortiz apologised to his opponent in the centre of the ring by touching gloves before Mayweather suddenly responded with a left hook and crunching right that dropped Ortiz to the canvas at 2:59.
Boos rang out from the crowd in the MGM Grand Garden Arena but Mayweather, who improved his professional career record to a perfect 42-0 with 26 knockouts, was unrepentant.
“I got hit with a dirty shot, I was protecting myself at all times,” the 34-year-old said ringside. “I hit him with a left and then a right hand, and that ended the bout.”
Asked to describe what happened as a replay of the final moments was shown on a giant television screen, Mayweather replied: “We touched gloves and we were back to fighting, then I threw the left and right hand and that’s all she wrote.
”In the ring, you just have to protect yourself at all times. I was victorious. If he wants a re-match, he can get a re-match.“
Mayweather, widely regarded as the best defensive fighter of his generation, then showered his interviewer with abuse and said he had never been given ”a fair shake“.
Here's the knock-out.
Now... Whatever you make of that, Mayweather is a dick!