Featherweight Daniel Ponce De Leon also earned a unanimous decision over Cornelius Lock on the undercard of Floyd Mayweather Jr.’s welterweight bout with Sugar Shane Mosley at the MGM Grand.

Alvarez (32-0-1, 24 KOs), a redheaded teenager from Guadalajara, Mexico, added to his growing reputation in his Las Vegas debut, rallying from a rough first round to outwork Cotto (31-2-1).

“It was just a matter of time,” Alvarez said. “The first round was a little scary, but I gained my confidence as the rounds went on.”

Cotto, the older brother of former champion welterweight Miguel Cotto, had Alvarez stunned on the ropes in the opening minutes. Alvarez shook it off and even sent Cotto stumbling to the canvas in the second round in a questionable knockdown.

Alvarez finally finished with a dominant ninth, repeatedly teeing off on Cotto’s defenseless head until referee Tony Weeks stopped it with 9 seconds left.

“Fighting in Vegas was a great atmosphere,” said Alvarez, who has won 28 straight fights since a draw when he was 15. “I want to come back and fight here again soon.”

Cotto was stopped for the first time in his career.