Marquez (left) floored Casamayor with a strong right in the 11th
Juan Manuel Marquez won his lightweight debut, stopping Joel Casamayor for the first time in his career in Las Vegas.
The Mexican moved up a weight as he looks to land a third bout with Manny Pacquiao and he began slowly, allowing his Cuban opponent to take charge.
As Marquez upped the tempo, Casamayor wasn't able to keep up and he was floored with a right in the 11th round.
Several big blows from Marquez followed and Casamayor went down for good with about seven seconds left in that round.
"I figured this would be a tough fight until the end, but I was a more intelligent fighter," Marquez said.
"I was watching out for myself, and I knew how to neutralise his left hand. I threw all my combinations, because I knew I couldn't win with just my right hand."
After defeating Diego Corales two years ago, the 37-year-old Casamayor has unofficially ruled the division but Marquez proved the extra weight did not both him.
"I fought as a champion, but Marquez was the best this night," Casamayor said.
"I deserve a rematch. Marquez knew how to control my punches. I had a bad night. I'm very emotional right now, but I honestly need the rematch."
On the same card, Vernon Forrest dominated Sergio Mora from start to finish to regain the WBC light-middleweight title with a unanimous 12-round decision.
Mora had surprised Forrest with a majority decision victory to claim the belt on 7 June.
But Forrest emphatically avenged that defeat, using a strong left jab and big overhand right as he earned scores of 118-109, 117-110 and 119-108 at the MGM Grand.
"I had to prove that I had a bad night (last time)," Forrest said. "Anybody can have a bad night. I controlled him with the jab. It was basic boxing."
Forrest improved to 41-3, with 29 knockouts. Mora fell to 21-1-1, with five wins inside the distance.