Three compelling bouts in eight years haven't resolved the rivalry between Pacquiao and Marquez, so they're stepping in the ring together for a fourth bout on Dec. 8 at the MGM Grand Garden in Las Vegas.

And this time, they're both looking for the only decisive ending to any fight: a stone-cold KO.

"I want to erase the doubt of the last three fights," Pacquiao said. "There's so many people still asking if I won the fights. I think to myself, 'Something is wrong. I have to do it again.' This time, I will train hard to put this fight up in the history of boxing. I want to make this fight short. I want to knock him out."

Although they're extending a rivalry to rare lengths in modern boxing, the fighters and promoters believe fans will warm to the matchup when they remember just how good the first three fights were.

Pacquiao and Marquez fought to a draw in 2004, while Pacquiao won by split decision in 2008 and again by majority decision last year. The bouts featured knockdowns, wild momentum swings and fascinating contrasts in technique - but Marquez and many fans still believe he won all three fights, while Pacquiao says he clearly won the last two.

"It's been an incredible ride," Top Rank promoter Todd duBoef said. "People ask me whether it's going to be hard to sell a fourth fight. Did you see the first 36 rounds? There wasn't a dull moment."

With a combination of unfinished business and unmatched financial reward, Pacquiao and Marquez both had plenty of incentive to get together again. They have fought at 125, 130 and 144 pounds, and their fourth fight will be a straight welterweight contest at 147.

Pacquiao (54-4-2, 38 KOs) is coming off his first loss since 2005, a wildly disputed decision to Timothy Bradley. Still stinging from that embarrassment, he says he's going back to the ferocious, relentless style of fighting that made him an eight-division champion. Pacquiao hasn't stopped an opponent in more than three years, a once-unthinkable drought for a relentless puncher.

"I want to be the other Manny Pacquiao, like when I was 24, 25 years old," Pacquiao said. "I want people who watch this fight to be satisfied. I don't care about a belt. I don't care about the money. I want the win."

Marquez (54-6-1, 39 KOs) has tested Pacquiao more than any opponent, with counterpunching skills and comprehensive boxing knowledge that can negate many of Pacquiao's strengths. But Pacquiao also is a nightmare matchup for Marquez, whose relentlessness hasn't been enough to overcome Pacquiao's once-in-a-generation combination of speed and strength.

While Pacquiao has a plan to recapture his best form, Marquez believes the only way to be sure he'll finally get his hand raised is to stop the Filipino congressman. Marquez swore off the rivalry and nearly retired in frustration immediately after Pacquiao's victory last fall, but agreed to return after a few months to cool down.

"I won all of the last three fights," Marquez said. "I would like the referee to raise my hand. Everybody knows I won the fights, and I don't know what happened with the judges. I have to take it out of the judges' hands this time."

Pacquiao won't train in Baguio, the high-altitude northern Philippines city where he has traditionally started training camp before heading to trainer Freddie Roach's gym for the final few weeks.

Instead, Pacquiao and Roach will be in Hollywood for their entire training camp, dramatically reducing the number of distractions for the Philippines' most famous person.

Pacquiao and Marquez will join the short list of rivalries that couldn't be contained by a mere trilogy, including Sugar Ray Robinson's six fights with Jake LaMotta and Robinson's four bouts with Gene Fullmer. More recently, Israel Vazquez's sensational rivalry with Rafael Marquez, Juan Manuel's brother, extended to four fights, concluding with Marquez's third-round stoppage of Vazquez in May 2010.

"I never thought it would go to four fights," Pacquiao said. "It's a first for me."

- AP