LAS VEGAS - Georges St. Pierre and B.J. Penn both talked of legacy and the desire to be known as the best fighter in the world. But Penn pointed to a more basic motivation driving him ahead of Saturday's showdown at UFC 94.

"I just want to kick his ass," the Hawaiian fighter said Wednesday at the pre-fight news conference.

Penn did just that in the first round the first time they met, at UFC 58 in March 2006. Penn used his striking skills to bust the Canadian open and by the end of the round, St. Pierre's face was a mask of blood. He was so befuddled he even headed to Penn's corner instead of his own when the round ended.

St. Pierre bounced back, gamely using his wrestling skills to take Penn down and suppress him the final two rounds. St. Pierre won a split decision, but it was hard to tell after the fight.

St. Pierre looked liked someone had taken a baseball bat to his shaven head, which was knotted with bumps all over. After being poked in the eye in the first round, he headed to hospital as a precautionary measure. Penn partied the night away, wondering what had just happened.

Almost three years later, they meet again -- this time at the MGM Grand Garden Arena. St. Pierre (17-2) holds the welterweight title at 170 pounds while Penn (13-4-1), a former 170-pound champ, holds the 155-pound belt. The fight will be contested at 170 pounds, with St. Pierre's title on the line.

They are fighting for a place in mixed martial arts history, vying with perhaps only UFC middleweight champion Anderson Silva and former Pride heavyweight title-holder Fedor Emelianenko to be the best of a generation of a new breed of MMA fighters who are skilled in all aspects of the game.

The first fight was a watershed for St. Pierre. He was cut open and hurt. And he came back.

Now the Montrealer wants to finish Penn -- and leave him in his tracks.

"I want to be the best fighter in the world, known as the best pound-for-pound (fighter) in the world," St. Pierre said. "And to do so, there is this guy B.J. Penn who's there and I need to beat him to be able to do that."

Both have to make 170 pounds at Friday's weigh-in and Penn -- at 171 pounds -- is pretty much there already. St. Pierre is bigger and will have to shed weight. His camp says he will probably be down to 175 or 176 pounds Thursday, squeezing out the final few pounds in the sauna.

St. Pierre, 27, competes at 170 but essentially only ever makes that weight for four or five hours around a fight weigh-in.

Come fight night, the Montrealer will be closer to 185 to 187 pounds when he enters the cage. Penn will still be around 170.

Does size matter? Penn was digging away at his rival when the two squared off on stage Wednesday to pose for photographers.

"B.J. said `You cut too much weight, Georges. You're going to be weak. You cut too much weight,"' recounted UFC president Dana White, who was between them. "Georges kept going `we'll see B.J. , we'll see.'

"That goes both ways. is B.J. too small? B.J. fought at 170 pounds before, B.J. fought at 185 pounds. I think it could go both ways," White said of the size disparity. "The weight-cutting isn't good for Georges and Georges being bigger than B.J. maybe isn't good for B.J. We'll find out Saturday night."

Of course, cutting weight is way of life in mixed martial arts. The idea is to be bigger than your opponent and St. Pierre trims down for every bout.

Penn, a natural talent who has rededicated himself to training in recent years, has enjoyed great success in his latest lease of life as a lightweight. White asked him to go back to 155 pounds, in essence forcing him to get into shape.

St. Pierre won the fashion war Wednesday, wearing a dapper pinstriped suit and tie. The ever-casual Penn wore his usual bjpenn.com T-shirt.

The 30-year-old Hawaiian was all-business.

"I'm here, I'm ready. Let's go," he said when called to the microphone.

His brother J.D. says Penn has his game face on.

"He's very focused. Usually he's having a little bit more fun. I think he's excited. I mean he's not showing it, but I think inside he's already ready to fight. He kind of turned this way a week ago."

St. Pierre was equally serious.

"This is one of those fights that's going to make history. I'm ready for it. I've never been so excited for a fight and I can't wait. I'm ready to go."