But Lesnar, a former World Wrestling Entertainment "champion," is used to playing a villain, and now he gets to do it in a real sport.
"I may come off as a cocky...but I'm just confident," the 6-foot-3, 265-pound Lesnar said after knocking down the 6-2, 220-pound Couture with a stiff right that he followed with more than a dozen blows to the former champion's head before referee Mario Yamasaki stopped the bout 3:07 into the round at MGM Grand Garden Arena.
"I just believe in hard work, and that it pays off."
Lesnar, 31, imposed his will throughout against Couture, 45, making the mixed martial arts legend appear rusty from the near 15-month layoff due to his contract squabble and lawsuit against UFC that the parties settled in September.
"I don't think the rust is an issue at all," Couture said after the defeat. "A big guy caught me with a big shot."
In only his fourth pro MMA fight, Lesnar strode into the octagon with Metallica's "Enter Sandman" playing before a capacity crowd—a bit of theatrics he was used to as a WWE star. His pre-fight appearance was roundly jeered by a crowd who clearly adore Couture, and Lesnar's popularity wasn't helped when he said to Couture in a videotaped appearance, "You're just too damn old to be here."
But the 2000 NCAA champion wrestler from the University of Minnesota backed up the talk.
He won the first round on all three judges' scorecards, reversing Couture's ploy to press him into the cage. Lesnar then pulled out an old wrestling move, shooting Couture and forcing the champion to do what he wanted to avoid, spending too much time underneath the bigger man.
Couture (16-9) tried a few submission tricks, knowing that's what got Lesnar in trouble two fights ago in a loss, but Lesnar's raw strength overcame the attempts.
In the second round, Lesnar smashed a right to Couture's head that he answered with a right that drew blood from Lesnar's right eyebrow. Couture charged to press the wounded Lesnar backward, but Lesnar moved away and delivered the telling right cross that struck Couture behind the ear.
The older champion was down, and Lesnar unleashed a flurry that almost went on too long as if Yamasaki felt he owed it to the champion to give a response that never came.
"I can't believe it," Lesnar said in slipping on his new belt. "Randy has a lot of [courage]...I have the ultimate respect."
Lesnar will now await the winner of the Dec. 27 fight between Antonio Rodrigo Nogueira and Frank Mir, who beat Lesnar earlier this year. Couture still has two fights left on his UFC contract, but didn't elaborate on his plans.