Four fights into his mixed martial arts career, Brock Lesnar is the UFC heavyweight champion.

The former NCAA wrestling champion, who cuts weight to pack his monstrous physique inside the heavyweight division’s 265-pound limit, stopped three-time UFC heavyweight titleholder Randy Couture on Saturday at the MGM Grand Garden Arena in Las Vegas.

At times it looked as if Couture was on his way to executing another brilliant game plan. Lesnar, who had a roughly 50-pound weight advantage, took down the champion in the first round with a double-leg. When he tried to move into the mount, though, Couture reversed. Lesnar soon scored his own reversal, but Couture was making the big man work hard, which could have paid off had the fight gone the full five rounds.

Despite his size advantage, Lesnar, 31, was not able to keep Couture down in the first period. Couture also punched better coming out of the clinches, though Lesnar kneed hard to his body and was clearly throwing the more powerful strikes.

Early in the second round, one of those strikes landed. Lesnar stepped in with a short but massive elbow, which wobbled Couture. The 45-year-old UFC legend’s experience moved the fight against the fence, where he seemed to recover.

As the round wore on, the fighters moved around the Octagon, picking their shots. The game seemed to suit Couture, who landed a right hand that cut Lesnar’s eye. Lesnar’s power proved the great equalizer, however, as a straight right collapsed Couture to the mat.

Referee Mario Yamasaki gave Couture a good deal of time to recover, but Lesnar’s follow-up hammerfists forced the stoppage at 3:07.

“I can’t believe it,” said Lesnar, now 3-1, of being the UFC heavyweight champion. “I can’t believe it.”

Couture, who fell to 16-9, said he had felt comfortable with the size difference and declined to attribute the loss to ring rust despite not fighting for more than a year.

“I felt good,” he said. “He’s just a big guy. He caught me with a big shot.”