Floyd Mayweather dethroned WBC welterweight champion Victor Ortiz with a controversial blind-side punch in the fourth round of their eventful title fight on Saturday in Las Vegas. Mayweather knocked out Ortiz at 2:59 of the fourth as he appeared to hit the champion and fellow American after a break in the fight with a left-right combination while Ortiz had his hands down in the centre of the ring. "We touched gloves and that is all she wrote," The still-unbeaten Mayweather said. "He did something dirty but we are not here to complain about what he did dirty or I did dirty." Referee Joe Cortez had stopped the fight in mid-round just seconds earlier to deduct a point from Ortiz for head-butting Mayweather in the corner during a vicious exchange of punches. Ortiz had just finished apologizing twice to Mayweather for the head butt when Mayweather surprised him with a short left. After the fight, Ortiz and his corner complained about the way the referee handled the final moments of the fight, but Ortiz also took some of the blame for not protecting himself. "It was called a break and I did as I was told," Ortiz said. "I looked at the ref and then boom! I was out. There was a little miscommunication by the ref. But I have no one to blame but myself. It was a learning experience." The MGM Hotel and Casino crowd booed the decision by the referee to give the victory to Mayweather, who had been winning the 147-pound fight by a slim margin through the first three rounds. The 34-year-old Mayweather is now 42-0 with 26 knockouts and looked impressive early despite taking 16 months off since his last fight. Ortiz, who came into the fight 14 pounds heavier than Mayweather, was coming off an impressive victory over Andre Berto. In the fourth round, the two fighters were locked in a corner exchanging blows when Ortiz thrust his head up and forward to hit Mayweather, who had his back to the ropes. Mayweather, who was not cut, immediately complained to the referee, who stopped the fight to deduct a point from Ortiz. Ortiz immediately apologized to Mayweather. "I am not a dirty fighter. I told Floyd I didn't mean to do that," Ortiz said. The fighters then moved to the centre of the ring and Ortiz put his arms around Mayweather and apologized a second time. Ortiz dropped his hands and stepped back as Mayweather caught him unaware with a quick left that stunned Ortiz before knocking him down with a straight right hand while the referee was looking the other way. "I got hit with a dirty shot. It is protect yourself at all time. I hit left hook, right hand ended the bout," Mayweather said. Ortiz had been fighting well up to that point, showing he was more than just Mayweather's tuneup for a possible fight with Filipino champ Manny Pacquiao next year. The 24-year-old Ortiz was fighting for the second time as a true welterweight as his record fell to 29-3-2 with 22 knockouts.