Filipino boxing icon Manny Pacquiao has little to prove after world titles in a record eight weight classes, but he carries an extra fire into Saturday's fight with Mexico's Juan Manuel Marquez.
Pacquiao, regarded as the world pound-for-pound king, will defend his World Boxing Organization title against Marquez after they fought to a 2004 draw and the Asian southpaw won a 2008 rematch by one point on one judge's scorecard.
Marquez taunted Pacquiao by saying he truly won both prior fights, inspiring "PacMan" to train like never before.
"This is one of the most important fights I have had," Pacquiao said. "This fight is really important to me because he says he won the first two fights. The question is what do I need to do to prove to people I won those fights."
A knockout would leave no doubt about which fighter is superior and both men have made changes to boost their chances of landing such a blow, Marquez adding muscle to boost his power and Pacquiao improving his footwork and right hand.
"With the aggressiveness Manny has had in training, I'm sure that's what he's looking for," Pacquiao trainer Freddie Roach said. "He has a little extra fire in him. His attitude is aggressive. He wants this fight badly.
"If Manny puts him down I don't think he will get up."
Pacquiao knocked down Marquez three times in the first round of their first fight but could not win and knocked him down in the third round of the rematch before barely escaping with the decision.
"I will go to my death and know that (Marquez) won that second fight," said Ignacio Beristain, Marquez's Hall of Fame trainer.
"I thought we won both of them," Roach said. "They were great fights."
Coming up empty twice against Pacquiao drives Marquez, who at 38 is six years older than the champion.
"I'm just as motivated as he is. I'm just as upset as he is because I deserved to win those two fights," Marquez said. "We're going to cure a lot of doubts."
Pacquiao, 53-3 with two drawn and 38 knockouts, has given Beristain sleepless nights studying video while trying to find a strategy to end the 14-fight win streak of "PacMan".
"You can lose a lot of sleep thinking about Manny Pacquiao," Beristain said. "He is going to come like a lion to this fight. He has the whole Philippines on his shoulders.
"Pacquiao has a great punch. Every minute of every round you are in danger if you get hit. We have worked on ways to avoid that. We talked a lot about being more explosive with his punches.
"Manny Pacquiao is a spectacular fighter. He likes to exchange. He always wants to fight. And he has found a guy who wants to stay in there and fight with him."
Marquez, 53-5 with one drawn and 39 knockouts, is more cautious about Pacquiao's improved right hand skill compared to the first two fights.
"In the past he didn't use his right hand," Marquez said. "Now he is using the right hand with speed. We have to be careful of both hands.
"His left hand was always the big one, the one we tried to nullify. I think his right is the better one now. You have to be careful knowing that he can knock you out with either hand."
But Beristain sees a possible edge for Marquez in Pacquiao's evolution.
"He's a more complete fighter. That helps us," Beristain said. "Before he was throwing wild punches from different angles. You never knew where he was coming from. Now you know right where he is coming from. You know what he's going to do."
Not so fast, Roach warns, citing Pacquiao's footwork.
"That's the key to the fight. That's the way to draw out Marquez," Roach said. "Manny's balance is better. He will throw the wild shot here and there. With Manny, it's hard to know when he's coming.
"It's because of the footwork. It's going to cause Marquez problems."
Marquez has added muscle but that has brought speed concerns, although Beristain says Marquez can trade blows or evade punches when necessary.
"I was very concerned at the beginning. I could see he was slowing down," Beristain said. "But in the last 15 days, he has picked it up. I'm very happy with how his speed is now. He's going to be fine for the fight."
Roach sees Marquez attacking fast to make the most of his extra size.
"Marquez is going to start quickly. Why else do you put that weight on?" Roach said. "He's going to try to get Manny out of there early.
"I don't see the muscle helping at all. I don't think muscle turns into punching power. Muscle doesn't make you a better puncher."