Floyd Mayweather claimed the WBC welterweight title with a controversial victory over Victor Ortiz in Las Vegas.
Mayweather knocked out Ortiz at the end of the fourth round to extend his unbeaten record to 42 fights.
But the 34-year-old looked to land the knockout blows while Ortiz was apologising after delivering a headbutt seconds earlier.
"I got hit by a dirty shot. The rule is protect yourself at all time," Mayweather insisted afterwards.
"We are not here to cry and complain about what he did or I did dirty."
After a quiet opening in Sunday's fight, it burst into life in the fourth round with Ortiz getting Mayweather on the ropes without quite being able to find a decisive blow.
Just before the bell Ortiz had Mayweather on the ropes for a third time, only to be penalised for a headbutt.
The 24-year-old then seemed to be apologising and was not protecting himself as Mayweather landed two punches - the second a big right hand - to end the fight.
Ortiz claimed he was hit by a punch before the referee had asked the fighters to continue.
"I took the break by the referee and I obeyed exactly as I was told," Ortiz said. "And then, boom, he blindsided me.
"There's two ways to look at it but as far as I was concerned I came here to entertain the fans and they were entertained.
"There was a miscommunication with the referee but nobody is perfect and this was a learning experience."
Mayweather had returned to the ring for the first time in 16 months.
Some critics had suggested the bout will be a 'tune-up' for Mayweather before an agreement is finally reached for a Manny Pacquiao fight next year.