Kermit Cintron discovered he could punch one rainy day as a high school freshman.

The former IBF welterweight titleholder was playing Frisbee football in his high school gym in Warminster, Pa. when an older, bigger classmate slammed him into the bleachers and injured his back.

In pain, he went to the nurse and had ice applied to the injury. His back was cold but his anger was hot. He ran into the kid after gym class.

“I threw one left hook and knocked him out cold. That’s when I knew I could punch,” said.

Not much has changed. More than a decade later, Cintron is still knocking people out with one of the most-feared punches in the sport, only now his victims are trained professional boxers.

His next victim? He hopes it’s Sergio Martinez on Saturday in Sunrise, Fla., for the interim WBC junior middleweight title, which is just another important fight for an important – if improbable – fighter.

Even now, eight-plus years into his pro career, Cintron marvels at his own success when he considers the highly unusual route he took.

Cintron was a child in his native Puerto Rico when his mother died of cancer. His father, unable to care for him and his siblings, sent him to live in the U.S. with his uncle, who happened to be former pro boxer Benjamin Serrano.

The boy didn’t get into boxing immediately, though. He hung out at the gym occasionally, hit the bags once in a while, got a taste of the sport, but never truly dived in as a youngster.

Instead, he became an all-around athlete who ended up a star wrestler, good enough to earn scholarship offers to two Big 10 schools.

“I wrestled, played football, ran track and played baseball,” Cintron said. “I was best in wrestling, though. I liked the one-on-one. I always loved it. Since the sixth grade, I always wrestled.”

Cintron (30-2, 27 knockouts) said he still gets the itch to flop onto the mat when wrestling season comes along. And one time, in the middle of his career, he gave in to his urge.

In 2003, he was 17-0 (with 16 knockouts) and a fast-rising prospect when he heard about an open wrestling tournament. He was set to fight journeyman Frankie Sanchez the following weekend but decided to take a chance.

With his handlers in the dark, he wrestled in the 152-pound class and had a ball. He also took first place.

“It felt good to get back to the old game, it felt really good,” he said.

That was a lark, however. Cintron had made his choice years earlier when he weighed the benefits of wrestling and the sport with which he was always fascinated, boxing, and decided that the latter made more sense financially.

He was 19 when he made the decision, though, an extremely advanced age to take up the sport in earnest.

Fortunately for Cintron, he had the discipline he adopted in wrestling, that natural punching power, some experience in the gym with his uncle and the ability to learn quickly.

“I just tried it out around that time and I felt I could be good at it,” he said. “I learned quickly. I know it was a risk (to take up boxing) but sometimes you have to take risks in life.

“ÔǪ Now, I look at it as an advantage over other boxers. I started late so my body isn’t beat up like some other guys.”

Evidently, Cintron was a fast learner. He ended up knocking out Sanchez that following weekend and kept rolling until he was 24-0 (with 23 KOs) to start his career.

And then, in 2005, he ran into a force more powerful than himself – Antonio Margarito, who took Cintron’s best punches, put him down four times and then knocked him out in the fifth round.

Devastating, yes, but only a minor slip-up in terms of his ascent in the sport. Two fights later, in October of 2006, the former wrestler knocked out Mark Suarez in the sixth round to win the vacant IBF welterweight belt.

“Honestly, I never imagined that would happen,” he said. “I really didn’t take it completely seriously until I signed with Main Events for my 12th or 13th fight. I was getting paid a few bucks to go in there and knock someone out.

“After that, I realized I could really take this to another level.”

Cintron continued to win until he ran into a familiar foe last April – Margarito.

No one will question Cintron’s courage after he climbed back into the ring with the man who had already beaten him silly. Courage doesn’t necessarily win fights, though.

The same thing happened: This time, Margarito took out Cintron with a vicious body punch in Round 6.

“All I know is that he can take a punch,” Cintron said of Margarito, who lost his license Tuesday in California for using illegal knuckle pads in his wraps. “He doesn’t have the best skills in boxing but he can take a punch and comes forward throwing punches in bunches. He wears you out. I hit him with everything I had and he just kept coming forward.

“I’m very competitive; I hate to lose. I wanted the rematch. I know I can beat him if I’m well prepared for the fight and well trained.”

The losses are indelible proof that Cintron can be overpowered unless it is somehow proved that Margarito used doctored gloves when they fought.

Still, promoter Lou DiBella, who signed Cintron after the second Margarito fight, said the losses must be perceived at least somewhat differently after the ruling of the California State Athletic Commission.

“I look at anyone who lost to that guy differently,” he said. “I wasn’t there; I didn’t promote him at that time. In the second loss, though, it appeared the punches really did damage. It was very similar to the way the (Miguel) Cotto fight (against Margarito) went down.

“The only two losses of his career were to Margarito. Unfortunately, there will always be doubt about anything that happened with Margarito in the past.”

Regardless, Cintron, 29, doesn’t appear to have lost any confidence.

He got back into the ring seven months later and easily outpointed capable Lovemore N’dou in an IBF title eliminator, which made him titleholder Joshua Clottey’s mandatory challenger.

However, with his wife and two kids in mind, he took a more-lucrative and higher-profile fight against Martinez at a heavier weight on short notice. He’ll earn more than twice as much as he would’ve made against Clottey.

Cintron has fought when he was heavier than 147 pounds but never in such a big fight.

In Martinez, an Argentine who fights out of Spain, he will be facing a slick-boxing southpaw and true junior middleweight who suffered his only loss to – you guessed it – Margarito.

As in the second Margarito fight, Cintron clearly isn’t afraid of a challenge.

“I’m here for the big fights,” he said. “The opportunity opened up and I took advantage of it. I had begun training for the Clottey fight (scheduled for a week later) so it’s not like I really started camp late.

“I feel strong. I don’t have a problem making 147 but I’m comfortable at 154. I’m just in the gym, training my ass off and not worrying about the weight.”

DiBella, who also promotes Martinez (44-1-1, 24 KOs), must stay neutral for this fight. However, he knows one thing: Anyone who has written off Cintron because of his losses to Margarito are making a big mistake.

Cintron, he said, can still compete with anyone in the world.

“He’s a dangerous guy no matter who he fights,” DiBella said. “On Saturday night, I expect him to make a real run.”

Michael Rosenthal can be reached at [email protected]