During the fourth round of last night's UFC lightweight title fight, commentator Joe Rogan asked the question quickly creeping into everyone's mind.

“;Who the hell is B.J. Penn going to fight next?”;

Granted, his bout with Diego Sanchez wasn't technically over, but the only drama left was to find out if Penn would become the first fighter to stop Sanchez in the allotted time limit.

He did exactly that, opening an ugly cut that forced the ringside doctor to stop the fight at 2:37 of the fifth round, giving Penn (15-5-1) a TKO over Sanchez in the main event of UFC 107 last night in Memphis, Tenn.

“;I was keeping my mouth shut, but I've always wanted to fight Diego Sanchez,”; Penn said after the fight.

Penn destroyed his fifth straight opponent at 155 pounds, as Sanchez joined the ranks of Kenny Florian, Sean Sherk, Joe Stevenson and Jens Pulver in failing to come close to beating the UFC's lightweight king.

The fight was so one-sided that it has opened the door for Penn to possibly move back up to 170 pounds, the division he lost to Georges St-Pierre in 11 months ago.

Sanchez (23-3) had never been knocked down in a fight in his career, but it took Penn less than a minute to floor Sanchez with a right hook. Penn followed up with a series of punches, but Sanchez escaped a possible quick end to the fight.

He never fully recovered, as Penn continued to take it to Sanchez, winning every round and nearly every exchange. Without a legitimate standing attack to counter Penn's efficient counter-punching, all Sanchez had left was a series of failed takedown attempts that Penn fought off over and over.

Penn turned up the heat in the final round and caught Sanchez with a kick to the head that opened up a gash similar to the cut he opened up on Stevenson at UFC 80. As soon as the fighters got to their feet, the referee immediately stopped the bout to check on Sanchez as blood poured down his face. It took the doctor mere seconds to stop the fight after looking at the cut that engulfed the entire left side of Sanchez's forehead.

The biggest question Penn will now face is whether he jumps up in weight or is content defending his lightweight belt. He said repeatedly during the week that he wants to fight St-Pierre a third time, but UFC President Dana White countered saying he'd have to beat one or two other guys at 170 before a fight with the champion.

That leaves possible opponents in Jon Fitch, Josh Koscheck or even a third fight with Matt Hughes should Penn and the UFC decide he has effectively cleaned out the 155-pound division.

On the undercard, Hilo's Shane Nelson lost by unanimous decision to Matt Wiman. Nelson lost 30-27 on all three scorecards and suffered his second straight defeat in the Octagon.