LAS VEGAS — Canada's Georges St-Pierre stunned BJ Penn — and thousands of Hawai'i fans — with a dominating victory in the main event of the UFC 94 mixed martial arts card tonight.

A capacity crowd of more than 17,000 at the MGM Grand Garden Arena watched St-Pierre beat Penn for the second time.

The bout was called after four rounds. A ringside doctor called it off after inspecting Penn in between the fourth and fifth rounds.

The bout was scheduled for five 5-minute rounds.

St-Pierre improved to 18-2 and defended his UFC welterweight (170 pounds) world championship.

Penn, who is still the UFC lightweight (155 pounds) world champion, dropped to 13-5-1.

St-Pierre dominated Penn like no other MMA competitor has before. St-Pierre used his strength to stay on top of Penn and land numerous strikes.

The first round was mostly about defense. St-Pierre attempted several single-leg takedowns, but Penn stayed on his feet.

Both fighters landed jab punches late in the round, but nothing significant.

The second round was where it turned and St-Pierre started to dominate.

He got Penn to the ground one minute into the round, then stayed in the top position for the next four minutes.

St-Pierre landed numerous devastating punches to Penn's face during those four minutes, and when the round ended, Penn staggered to his corner.

By the third round, blood was pouring out of Penn's nose. St-Pierre was then able to take Penn down again and continue the assault on the ground.

Early in the fourth round, St-Pierre pinned Penn's arms to the mat and proceeded to land several unanswered elbows and punches to Penn's face. By then the outcome was inevitable, and the only question left was whether Penn could last the entire five rounds.

He couldn't, according to the doctor.

In the semi-main event, Lyoto Machida remained undefeated in the 205-pound division with a knockout victory over fellow Brazilian Thiago Silva.

Machida landed a devastating right hand to the head of Silva, who was lying on his back when he received the punch. It came just before the horn sounded to end the five-minute first round.

Both fighters entered the bout undefeated. Machida is now 14-0 and considered one of the top contenders to face light heavyweight champion Rashad Evans.

Silva dropped to 13-1.

In the earlier bouts:

205 pounds: Jon Jones def. Stephan Bonnar by unanimous decision.

170: Karo Parisyan def. Dong Hyun Kim by split decision.

155: Clay Guida def. Nate Diaz by split decision.

170: Jon Fitch def. Akihiro Gono by unanimous decision.

155: Thiago Tavares def. Manvel Gamburyan by unanimous decision.

170: John Howard def. Chris Wilson by split decision.

205: Jake O'Brien def. Christian Wellisch by split decision.

170: Dan Cramer def. Matt Arroyo by split decision.

Reach Dayton Morinaga at dmorinaga@honoluluadvertiser.com or 535-8101.