LAS VEGAS — Brock Lesnar stopped heavyweight champion Randy Couture in the second round of their fight at UFC 91 tonight, earning a technical knockout when the referee stepped in as he kept punching after Couture went to the ground.

Lesnar, a former NCAA wrestling champion and WWE star, overwhelmed the 45-year-old Couture in just his fourth pro fight. Lesnar sent him to the canvas in the second round with a glancing blow that landed behind the ear and wasted no time in landing numerous hammer fists. Couture was never able to recover and the fight was stopped at 3:07.

"It sounds really good," Lesnar said after being introduced as the new heavyweight champion. "I can't believe it.

"I just believe in hard work and it pays off."

The 6-foot-2, 220-pound Couture was at a decided physical disadvantage against the 6-3, 275-pound Lesnar (3-1), who is 31 and at the start of his pro career. Couture (16-9) also had hadn't fought for 15 months, stepping away when he became fed up with contract squabbles and his inability to land the big fights he really craved.

Couture wasn't sure about his future after the bout.

"I still feel like I'm getting better as a fighter so we'll see," said Couture, who has two fights remaining on his contract.

In the co-main event, Kenny Florian moved one step closer to a title shot when he forced Joe Stevenson to submit in the first round.

"This is exactly what I wanted," Florian said. "I wanted to come out and make a statement."

Florian almost immediately shifted his focus to lightweight champion B.J Penn.

"I want B.J.'s belt," he said. "I consider you a master and now it's time to kill the master."

Also on the card, Dustin Hazelett defeated Tamdan McCrory via arm-bar at 3:59 of the first round. Gabriel Gonzaga, the last man to face Couture, earned a TKO over Josh Hendricks, dropping him with a hard right before the referee stopped the fight at 1:01 of the first round. Demian Maia made short work of Nate Quarry to remain undefeated, getting him to submit at 2:13 of the first round.

In the preliminary bouts, Aaron Riley beat Jorge Gurgel in an unanimous decision. Jeremy Stephens knocked out UFC newcomer Rafael dos Anjos 39 seconds into the third round. Mark Bocek got Alvin Robinson to submit in the third and Matt Brown forced Ryan Thomas to tap with an arm-bar 57 seconds into the second round.