LAS VEGAS — Hilo's BJ Penn (13-4-1) legitimized his UFC lightweight title with a victory over former champion Sean "The Muscle Shark" Sherk (32-3-1) in the third round at UFC 84 last night at the MGM Grand Garden Arena.

Veteran referee Mario Yamasaki determined that Sherk was unable to continue after a Penn flurry at the cage that left Sherk on the mat at the end of the third round.

Penn called the victory, "Satisfying! Very satisfying." In the post-fight press conference, Penn told reporters, "I felt really good and in really great shape. I think I got things under control these days."

The three five-minute rounds were fought mostly standing up, resembling a boxing match.

Penn worked a slicker boxing style set up with a long, snapping jab. Sherk threw left and right hooks in a stiff, technical style.

Penn established his jab in the first round as Sherk pressured. Sherk clinched to muffle the reach advantage of "The Prodigy," but Penn landed an uppercut to break the clinch.

Sherk attempted to counter with his own jab, but threw it short as Penn followed with a left. Sherk clinched again, but Penn threw him off.

Sherk began round two with a cut under his right eye from Penn's relentless jabbing. He shrugged his shoulders before the bell, disappointed with his first-round performance.

But the Minnesotan took the fight to Penn in the second round. Penn clinched inside Sherk's left/right hook combo and hammered knees to his body.

Again, Penn clinched, but Sherk overwhelmed him with a flurry. Penn pushed out of the clinch.

Unable to solve Penn's boxing, Sherk began to test his leg kick. Penn answered that with a left hook.

Indeed, Penn had all the answer last night.

Sherk began the third round with another cut under his left eye. Before the bell rang, he sized himself up on the screen above the octagon.

Increasingly desperate to overcome Penn's reach advantage, Sherk began lunging with his punches.

Penn timed an uppercut as Sherk lunged into a punch, sending Sherk backpedaling to the cage. Penn followed with a flying knee that landed squarely on Sherk's jaw. Penn then mounted and pounded Sherk to the bell.

"He got caught," Penn said. "I hit him and I knew the fight was done when he put his hands up and looked at me like, 'Yeah, you got me. It's over.' "

The referee stopped the action at the bell, but determined Sherk was no longer fit to fight another round after Sherk remained on his knees and awarded Penn the TKO.

When asked if the stoppage was justified, Sherk replied: "I'm not real sure. I didn't even hear the bell ring."

"I was surprised (Sherk wanted to box)," Penn said. "I thought he was going to try to use his combinations to set up his takedowns."

After the fight, Penn asked the 15,000 in attendance: "You want GSP? You want BJ Penn to fight GSP?"

He was referring to UFC welterweight champion Georges St. Pierre (16-2), who won a split decision over Penn in 2006.

UFC President Dana White told reporters that Penn has at least one more fight before "clearing out the lightweight division" and earning a rematch with the 170-pound St. Pierre.

White is thinking ahead to the winner of the August fight between "KenFlo" Kenny Florian (9-3) and "El Matador" Roger Huerta (20-1-1).

The Penn-Sherk fight was a grudge match fought against the backdrop of a positive steroid test last July that cost Sherk his title. Sherk denied cheating, but Penn wasted few opportunities to taunt him as a cheat. And the crowd chanted "Steroids, Steroids" during the bout.

Earlier, Wanderlei (The Axe Murderer) Silva scored a knockout win. Fellow light-heavyweight Tito (The Huntington Beach Bad Boy) Ortiz lost by unanimous decision.

The Associated Press contributed to this report.