Despite being in position to challenge for the WBA light middleweight title, Mosley decided to return to the welterweight division. In January 2007, Mosley and former WBA welterweight champion Lus Collazo (27-2, 13 KOs) announced that they would meet on February 10, 2007 at the Mandalay Bay Hotel in Las Vegas, Nevada in a fight billed as "Battle at the Bay". The interim WBC welterweight title was at stake because then full WBC champion Floyd Mayweather Jr. had planned to move up to light middleweight to challenge Oscar De La Hoya. There was no confirmation as to whether Mayweather would stay at 154 or return to 147 after that bout. Mosley knocked Collazo down in round 11 with a right hook and went on to win a fairly one sided fight with judges scoring the fight 119-108, 118-109 and 118-109. Mosley was guaranteed $1 million for the fight.