LAS VEGAS » Georges St-Pierre fell over attempting his trademark post-fight somersault.

It proved he's not perfect after all.

Everything up to that point couldn't have gone any better for the welterweight champion, who cemented his place as one of the all-time greats with a brutally dominant TKO victory over B.J. Penn in the main event of UFC 94 at the MGM Grand Garden Arena in front of a capacity crowd of 14,885.

Referee Herb Dean stopped the fight between the fourth and fifth rounds on the advice of the ringside doctor, who ruled Penn couldn't continue. Penn was not available for comment after the fight after being taken straight to a local hospital.

“;Georges St-Pierre completely dominated the fight,”; UFC President Dana White said. “;He said he was going to do it and he did it.”;

In the first fight between the two nearly three years ago, St-Pierre won a competitive split decision, leaving much speculation as to who really is the better fighter.

There's no doubt about it now.

“;This time I really wanted to take him out and I'm glad that I did it,”; said St-Pierre, who raised his record to 18-2. Penn fell to 13-5-1, including three straight losses at 170 pounds.

Penn weighed in at 168 pounds to St-Pierre's 170. But by the time the two stepped into the Octagon, St-Pierre had his weight up to 185-187 pounds, by his own estimate—and the difference in size showed.

St-Pierre showed an amazing ability to control Penn, a world champion in Brazilian jiu-jitsu, on the ground, beating him up from all angles in the second through fourth rounds.

Penn was able to stuff St-Pierre's early single-leg takedowns, but after hanging with the champion through a close first round, he had no answer for St-Pierre's relentless attack.

“;I knew I broke him mentally after the first round,”; St-Pierre said.

After the first fight, it seemed Penn's striking was superior to St-Pierre's. That wasn't the case last night as St-Pierre showed great improvement in his standup game, battering Penn with jabs, especially in the later rounds.

The fight was even after the first round as St-Pierre tried over and over to take Penn down with the single leg, but Penn escaped all three times.

It took St-Pierre a minute into Round 2 to finally secure the takedown, and once he did, the ground-and-pound beating was on.

It became a matter of survival for Penn, as St-Pierre tore through him over the next 15 minutes. Two or three times in the fourth round, it looked as if Dean was close to stopping the fight, but Penn hung in there until the doctor ruled he shouldn't continue to compete.

“;I was trying to finish him bad, but he always survived,”; St-Pierre said.

It's a deflating loss for Penn, whose stardom in mixed martial arts is largely due to the fact he had never been soundly beaten. Of the four men to beat him, he's avenged two of those losses, and a third was against undefeated 205-pounder Lyoto Machida in Japan, whom he took to a decision.

St-Pierre tore him apart in a way most thought unimaginable. Penn's aspirations to fight anybody at any weight took a major hit, as he'll probably be stuck in the 155-pound weight class for a long time. He's still a champion and will have to get over last night's performance quickly to get ready to defend his lightweight title against top contender Kenny Florian as soon as he's physically fit.