“In the third round of my fight, I mean, Marquez went down hard. But he gets right back up, shakes it off and he’s right back in there again,” said Bayless.

“And I believe that in that fight, Marquez staggered Pacquiao a few times, and Pacquiao ain’t going to go down either. It’s just heart. These guys have just got a lot of heart.”

On Saturday night, the 32-year-old Pacquiao (53-3-2, 38 KOs) returns to the site of their first meeting, where the eight-division titleholder will try to defend his WBO welterweigbht belt against the 38-year-old Marquez (53-5-1, 39 KOs), owner of the WBO and WBA lightweight crowns.

Ranked No. 1 pound-for-pound by THE RING, Pacquiao is 5-0, with two knockouts fighting at 147 pounds or higher, even capturing the since-vacated WBC junior middleweight belt with a unanimous decision over Antonio Margarito in November.

Marquez, meanwhile, is in only his second his second-ever 147-pound clash, having been easily decision by current WBC welterweight beltholder Floyd Mayweather Jr. (42-0, 26 KOs) during his division debut in September of 2009.

Many observers believe that Pacquiao’s two-fisted power coupled with his experience at the higher weight will be too much for Marquez. Pacquiao’s corner man, five-time Trainer of the Year, Freddie Roach, for example, has predicted that Pacquiao could end the fight as early as the sixth round.

Bayless is not so sure. Asked if he sees Marquez being knocked out, Bayless said, “No I don’t.”

“I know that all of the leaders talking and saying that there is going to be a knockout, but I see it possibly going the distance because they’ve fought for 24 rounds now,” said Bayless.

“And Marquez got knocked down four times in 24 rounds and both fights went the distance. Having been in there with these two guys, there is something about that heart and their desire.”

Bayless compared Pacquiao-Marquez favorably to the Muhammad Ali-Joe Frazier trilogy.

“Today, I watched the Thrilla in Manila [Ali-Frazier I] on HBO, and [promoter] Butch Lewis kept saying, ‘you know, what makes a guy like Joe Frazier, after all of the hits that he was taking, keep wanting to fight?’ Well it’s Heart,” said Bayless.

“It’s their heart. Their special. And only certain guys have it. And both of those guys [Pacquiao and Marquez] have it. So that’s why I say that it’s going to be a hard, gruelling fight that will go the distance.”