It happens often in boxing: a great fighter, no longer in his prime, agrees to take one bout too many. And Sunday night in Las Vegas it happened again, this time in mixed martial arts at The Ultimate Fighter 19 Finale.

Former UFC lightweight/welterweight champion B.J. Penn dropped to featherweight for a third battle with fellow ex-lightweight titleholder Frankie Edgar. It was Penn's debut at 145 pounds and it wasn't a pretty scene.

Edgar took charge from the opening horn and never let up en route to a third-round TKO that concluded at the 4:16 mark. Afterward a bruised, battered and emotional Penn announced his retirement.

Penn admitted that upon reflection this was a fight he probably should not have accepted.

"This is the end," a teary-eyed Penn said. "I'm thinking to myself, 'Why did you step back into the Octagon after the beating Rory MacDonald gave you?' The reason is if I didn't make this night happen for myself, I would have always wondered. I guess I needed some closure."

MacDonald defeated Penn by a lopsided unanimous decision when the two met in December 2012. Penn seriously contemplated retirement at that time, but reconsidered when the offer to fight Edgar once more was presented.

Edgar not only shut the door on Penn's career; he threw the key away. At no point in the fight did Penn threaten to exact revenge on Edgar, who had won each of their first two meetings [at 155] by unanimous decision.

While standing, Edgar [17-4-1] used his solid footwork and head movement to get inside, throw body shots and slip out to avoid Penn's power shots. Edgar had seemingly little trouble taking Penn to the ground and scoring with strikes.

There was nothing Penn, 35, could do to slow Edgar's assault. He showed very few signs of the fighter destined for the UFC Hall of Fame. One of the greatest fighters in MMA history was clearly no more.

"I have nothing but respect for B.J." Edgar said. "It was a bittersweet victory. I said I wanted to finish him, but I almost feel about about it."

Penn the first undisputed UFC welterweight champion, finishes his career on a three-fight losing skid and a professional record of 16-10-2.