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."
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