The fight was for the UFC welterweight title with Georges St. Pierre defending against former welterweight champion and current lightweight champion B.J. Penn. The fight was marred afterwards by allegations from Penn that St. Pierre had used grease (Vaseline) to avoid Penn getting a hold of him on the ground, and allowing St. Pierre to pass his guard easily and land more punishment.
The first round began and they clinched early. They exchanged knees. St. Pierre landed a shot. St. Pierre began looking for a single but Penn defended. Penn landed a shot. The crowd chanted 'BJ'. St. Pierre had a leg but Penn still stood. St. Pierre landed a knee while still holding the leg. Penn landed a right hand over the top. Herb Dean ordered the fighters to work in the clinch and they broke. Penn landed a good shot, responded by St. Pierre over the top. St. Pierre landed a jab. They clinched once more and St. Pierre landed a knee. St. Pierre grabbed the leg once more but Penn just would not go down to the ground. Penn landed an over-the-top shot to the side of St. Pierre's head. The crowd now began chanting 'GSP'. Penn landed a good shot. They clinched once more. St. Pierre landed a knee. They broke and Penn landed a shot. Twenty seconds remained. St. Pierre landed a leg kick. They clinched once more, broke, and St. Pierre landed a shot and the first round ended. It was unclear who won the first round.
The second round began. Penn landed a jab. St. Pierre landed a body shot. Penn pushed forward and landed a good left. St. Pierre clenched and landed the knees. He landed a good over-the-top shot. Rogan commented that Penn didn't even look a 'little bit tired'. St. Pierre secured the takedown but Penn worked the rubber guard. He went back to full guard. Penn continued attempting to work the rubber guard but his legs kept slipping strangely on St. Pierre's shoulders. The crowd chanted 'BJ'. St. Pierre moved to half guard and then side control, ground-and-pounding viciously. St. Pierre landed a knee to Penn's midsection. St. Pierre continued ground-and-pounding. Penn pulled guard again somehow. St. Pierre half-stood and postured, throwing hard strikes from the top. St. Pierre landed two hard elbows from the top. Penn tried for another rubber guard. St. Pierre stood, landed hard shots from the top, went back to side control. Penn shoved back to full guard. The crowd chanted 'GSP'. Penn was cut underneath his eye at this point. St. Pierre continued landing hard shots from the top. Both commentators thought that the second round decisively belonged to the champion himself, St. Pierre. Penn looked exhausted at this point.
The third round began. St. Pierre landed a jab and then a leg kick, followed by a Superman punch. Penn was bleeding on his nose now, from the Superman punch. St. Pierre landed a hard shot and then a jab and then another. They clinched after Penn landed a jab. The crowd chanted 'BJ'. St. Pierre went for the takedown. Penn pulled half-guard. The crowd chanted 'GSP'. St. Pierre landed hard shots from the top. Penn struggled for the high guard again, but it just slid off St. Pierre's shoulders. Rogan commented that St. Pierre was slippery because of sweat. Penn got a leg and stood up and they stood against the cage. St. Pierre landed a hard knee. St. Pierre got the takedown against the cage and Penn pulled guard. One minute remained in the third round. St. Pierre landed a hard shot from the top. Penn looked dazed. The round came to conclusion. St. Pierre had decisively won the round.
The fourth round began. St. Pierre landed a leg kick. St. Pierre landed a jab and then a kick. St. Pierre got the takedown and moved quickly into half-guard and then into side control quickly. St. Pierre landed hard elbows from the top. St. Pierre landed numerous shots to the back of the head and was warned for it. He did it again, and was warned once more. St. Pierre landed an elbow. St. Pierre attempted to transition to mount. Herb Dean warned St. Pierre that he would stand the fight up if St. Pierre hit the back of the head anymore. Penn moved it back to half-guard but St. Pierre pushed back to side control. The crowd chanted 'GSP'. Herb Dean said 'BJ, you have to do something. You need to fight back.' Penn shoved it back to half-guard somehow. St. Pierre got back to side control. St. Pierre got the mount and Penn attempted to roll out but was unsuccessful. The crowd roared. St. Pierre went back to side control. One minute remained. St. Pierre landed hard elbows from the top and battered Penn. The ground-and-pound continued. Herb Dean warned that he was about to stop the fight. Penn fought as hard as he could to defend. St. Pierre landed a hard elbow and the fourth round ended. Penn looked dazed. Penn leaned against the fence, hurt badly. Penn's corner threw in the towel and the crowd roared. St. Pierre came over to hug and console BJ Penn. St. Pierre had won the fight by corner stoppage.