How does Juan Manuel Marquez do it?

The 37-year-old lightweight champion is 57 bouts into his 17¾-year pro career and he still appears to be on top of his game.

Bernard Hopkins is older and still considered world class, but the 45-year-old former middleweight and light heavyweight champ looked ordinary in his last two bouts and is rarely in the kind of exciting ring battles that Marquez has often engaged in.

Sergio Martinez, who is close to Marquez’s age, is an elite boxer who makes for thrilling fights but the 35-year-old middleweight champ got a late start in the sport and has only recently come into his own.

Marquez (51-5-1, 37 knockouts), who defends his RING title against No. 1 contender Michael Katsidis on Saturday in Las Vegas on HBO, is the sport’s oldest fighter on the pound-for-pound list. Very few active fighters have been world class as long or longer than the Mexican veteran.

What’s the secret to his longevity?

“I don’t have the words to explain it,” Marquez said at a recent media workout in Los Angeles, “but much of it has to do with how I prepare for my fights.”

John Beyrooty, the former publicist for the now-defunct Forum Boxing, Inc., which promoted Marquez’s first 33 bouts, believes the fighter’s Spartan preparation is what makes him special.

“His attitude and determination and work ethic always stood out to me,” said Beyrooty, who is now a publicist for Brener Zwikel and Associates. “It’s his maturity, in and out of the ring, that separates him from most fighters. Marquez treated his career from day one the way all fighters should, but don’t. A boxer’s career is a finite thing, so every day of a prize fighter’s career is important. He got that at the beginning. He understood what being a pro athlete was about, which could be a reason that he’s still going strong.

“One thing about Juan Manuel and his brother Rafael is that you always knew they were living clean between fights. You knew they were always dedicated, like most of Nacho Beristain’s fighters. Nacho’s fighters were usually very business minded; they did what they were supposed to. They didn’t wander off the path at all.”

Still, Beyrooty is amazed that Marquez remains on the path more than 15 years after the Mexico City native first fought on a Forum Boxing card.

“I think I first saw him fight in January of 1995, his first bout at the Great Western Forum (in Inglewood, Calif.), or maybe it was in December of 1994 on a card we did at Caesars Palace (Las Vegas),” said Beyrooty. “Did I think he would still be fighting in 2010? Do the math. The first time I saw him he was 21 or 22 years old fighting at featherweight. The answer is no. I say that because I don’t expect any young fighter who campaigns around featherweight and super featherweight in their late teens and early 20s to still be around in their late 30s.

“It wasn’t that long ago when fighting in your late 30s meant you were through as a world-class fighter, but now it’s no big deal, it’s nothing.”

Fighters such as Hopkins and Marquez, both master technicians who are dedicated to their craft and conditioning, helped change the way fans view age in boxing.

Beyrooty has no doubt that Marquez’s precise counter-punching technique added years to his career while more popular peers, such as Marco Antonio Barrera and Erik Morales, may have shortened their primes with their aggressive styles.

“It wasn’t like he was special in any one area in those early years,” Beyrooty said. “He wasn’t the fastest or the slickest boxer. He wasn’t a ferocious puncher, but he also wasn’t what folks would call a ‘typical Mexican fighter.’ He wasn’t a face-first brawler. He was smart, and you could tell there was always some kind of plan that he was executing during the fight.

“Some fans didn’t like the way Marquez fought. Beristain was often criticized back then for making his fighters boring. I thought Marquez made for compelling fights because he usually closed the show after breaking his opponents down, but some thought he waited too long. Some thought he was too careful.”

Marquez was considered a pure counter puncher when he first entered THE RING’s ratings at featherweight in late 1996. Some believe his patient style resulted in his first loss, a unanimous decision to Freddie Norwood in 1999.

It would be 6¾ years before Marquez lost another fight. Marquez won two featherweight titles and held Manny Pacquiao to a thrilling draw that earned him pound-for-pound status during this time span, but he was still unsatisfied with the sport.

Frustrated with Forum Boxing’s inability to secure a title shot with Naseem Hamed and their seeming unwillingness to make a fight with Barrera, who the L.A.-based outfit also promoted, Marquez signed with Top Rank in early 2000 in hopes that the promotional company would match him with Morales, the Mexican star of their stable.

It never happened.

Marquez thought he earned a showdown with ‘El Terrible’ with his courageous showing against Pacquiao. He was demoralized when Top Rank instead matched Morales with the emerging Filipino star in early 2005, so much so that he briefly considered retirement.

“There was a moment when I thought about quitting,” Marquez said. “I signed with Top Rank with the goal of fighting Erik Morales, but they wouldn’t let me fight him. I thought I was wasting my time in the sport.”

To be fair to Top Rank, the Las Vegas-based company kept Marquez busy, moved him up the alphabet rankings and landed him two title fights (against Manuel Medina and Derrick Gainer) before securing the opportunity against Pacquiao on HBO. Marquez, through Beristain, turned down an immediate rematch with Pacquiao (for a reported $750,000) before Top Rank made Morales-Pacquiao I at junior lightweight.

Marquez, however, had a different perspective of how Top Rank handled his career.

“I felt like they were playing with my career,” he said. “I felt like I wasted five years of my life with them.”

Marquez’s career hit rock bottom when he lost a controversial decision to Chris John in the featherweight beltholder’s native Indonesia in early 2006.

However, beyond Marquez’s dedication and technical skills is probably the real reason for his longevity — his burning desire to succeed.

Despite ups and downs with the business side of the sport and setbacks in the ring, Marquez refuses to quit.

“He’ll never give up,” Beyrooty said. “We learned that in the first Pacquiao fight. He got up from three knockdowns in the first round and managed to earn a draw in a grueling fight. Sometimes a fighter takes a beating, and even if he make a fight out of it, down the line it causes him to lose something. But this guy hasn’t really slowed down. It seems like Marquez is fighting better than ever.

“He got up from a knockdown against Barrera and out-boxed one of the smartest fighters I’ve seen in any weight class. He gave Pacquiao hell in their rematch and probably deserved to win that fight. He outclassed Juan Diaz twice. He knocked out Joel Casamayor, who I thought was a great fighter. He completely dominated Rocky Juarez. The only guy to really beat Marquez is Mayweather, but nobody looks good against Floyd, and that fight was at welterweight! He weighed 134¾ pounds against Diaz before the Mayweather fight. He weighed 133¾ pounds for the Diaz rematch after the Mayweather fight. It takes a very special athlete to go up and down in weight at his age but still perform at the level he does.”

Marquez is special enough to be considered a 3-to-1 favorite to beat Katsidis (27-2, 25 KOs) despite being the older, smaller man.

However, the veteran isn’t resting on his laurels. Marquez views the rugged, powerful pressure fighter from Australia as a threat even though he has knocked out the two fighters who have defeated Katsidis (Diaz and Casamayor).

“I’m not thinking about those fights,” Marquez said. “Katsidis is a different fighter now. He learned from his losses to Casamayor and Diaz. I worry about everything he brings, not just his pressure or his strength. He’s a complete fighter.

“I’m going to have to use my experience and try to dictate the fight with my jab, combinations, footwork and counter punches.”

If Marquez beats Katsidis, he says he will pursue a third fight with Pacquiao, even if it means venturing back to the welterweight division. He also covets a title in a fourth division, presumably at junior welterweight.

“I want to make history,” said Marquez, who has won titles as featherweight, junior lightweight and lightweight. No fighter from Mexico has ever won titles in four divisions.

Marquez wants to be the first, and Golden Boy Promotions, which has promoted him since late 2006, will try to deliver the opportunity by matching the future hall of famer with the winner of either the Amir Khan-Marcos Maidana fight or the Tim Bradley-Devon Alexander showdown sometime in 2011.

“I’ll be watching those fights,” Marquez said of the two 140-pound title bouts, “and not just because I could fight the winners. I like to watch good fights. I love boxing.”

Beyrooty says boxing should love Marquez while he’s still around.

“Despite all the accolades we’ve given him, I still say that Marquez is underrated,” he said. “Nobody is calling him ‘JC Superstar,’ but his body of work is fairly astounding. He’s never ducked anybody. He’s fought everybody from 126 to 147 pounds that was willing to fight him and he seldom had a bad night. The fact that Erik Morales, who many say is great, never fought Juan Manuel Marquez may say more than anything we can about him.

“It’s a pleasure and an honor to know a guy like Marquez, and boxing fans should be grateful that there are fighters like him still around. It’s too bad there’s not more.”