LAS VEGAS – Juan Manuel Marquez’s four-year war with Manny Pacquiao continued Saturday night and the latest collateral damage was felt intensely by Joel Casamayor.

The RING Magazine and linear lightweight champion had spent most of his career in grueling hand-to-hand combat himself, having faced down the heaviest handed punchers in the junior lightweight and lightweight divisions without often taking a backward step. That all changed Saturday at the MGM Grand Garden Arena when Marquez overwhelmed the 37-year-old champion, negating his right hook and ultimately devastating him with two crushing right hands of his own that twice sent Casamayor to the floor before referee Tony Weeks stopped the bout at 2:55 of the 11th round.

By then Marquez had done what no other 130 or 135 pound fighter had often succeeded in doing. He had the crafty Cuban in almost constant retreat after the opening rounds, content most often to circle and sit on his back foot searching for openings Marquez refused to give him once he figured out how to avoid Casamayor’s straight left hand and a sweeping right hook Casamayor’s trainer, Nacho Beristain, had claimed was the only punch he was particularly concerned with.

Marquez had challenged Casamayor as the next step in his ongoing pursuit of Pacquiao. They have fought evenly twice in the past four years, their bouts ending in a draw and a controversial split decision for Pacquiao in March that still eats at Marquez. Much to his surprise, he was headed to another potentially suspect decision when he suddenly exploded a textbook perfect right hand off the chin of Casamayor and sent him tumbling to the floor with his body twisted in an odd shape and a stunned look on his face.

Casamayor got up but he was clearly without all his faculties. He held his hands high when asked if he could continue but the blank look on his face and his wobbly legs were the truth squad. Marquez sensed it immediately and moved forward with the look of a cold-blooded assassin on his face. Soon after, Casamayor was on the floor again with no reason to continue but his own fierce pride, the pride of a man who had never been stopped until he ran into Marquez.

Later Casamayor would claim he was off-balance rather than hurt but the fact is he was off-balance because he was hurt. And badly so, even before that late juncture.

Early in the fight Marquez (49-4-1, 36 KO) had been patiently studying Casamayor, judging his hand speed and what the launching pad was for that dangerous right and straight left. He paid for his education as Casamayor drilled him with the hook and a series of straight lefts that helped him take command of the early rounds.

But by the fight’s midpoint, Marquez had figured out how to negate the hook and slip the straight left and was either countering with his own right hand or beating Casamayor (36-4-1, 22 KO) to the punch on the inside. As time wore on and Marquez kept up the pace he seemed to wear down the older champion, slicing open a large gash in his right eye lid and bloodying his nose so completely it appeared to begin to affect his breathing.

As Marquez began to land the heavier blows and more consistently win the exchanges, Casamayor began to retreat, edging back warily and circling like a wise hunter looking to set a trap for a force he could not quite control on his own.

But Marquez is one of the most proficiently sound technicians in the sport, a fighter who follows closely the instruction of his trainer and creator, Nacho Beristain. Together they had devised a way to take the straight left hand away from Casamayor, the same straight left that had floored Marquez four times in the two Pacquiao fights, while also negating his right hook and creating opportunities both to counter punch him and land his own slamming rights.

“I figures this would be a tough fight to the end but I was the more intelligent fighter,’’ Marquez insisted. “I knew how to neutralize his left hand and I threw a lot of combinations because I knew I couldn’t win with just one punch at a time.’’

“Combinations, combinations!’’ was what Beristain was hollering at Marquez between nearly every round, even after ones in which he had seemed to dominate the champion by throwing them. Yet at the time of the stoppage two of the three judges had the fight even and the third had Marquez leading 97-93, a score that seemed  more in keeping with what was going on inside the ring.

Marquez admitted during the week that one of the reasons he has lately changed from a counter puncher content to follow where others lead into a more aggressive fighter was in reaction to Nevada judges who seem unable to understand how he goes about his work inside the ring. Saturday night they didn’t seem to get it yet but Juan Manuel Marquez took it out of their hands by using his own to break down and finally knock down an opponent who had never been stopped in a 15-year professional career.

In the end, Juan Manuel Marquez seemed to say that Manny Pacquiao can go off in pursuit of Oscar De La Hoya if he wants, as he plans to do Dec. 6, but when he’s done he’ll be waiting for him.

In the semi-main event, Vernon Forrest delivered on a promise to turn back the clock and remind the boxing world which was dismissing him as a shot fighter who he was.

“Now you see the difference between a contender and a real champion,’’ Forrest (41-3, 29 KO) said after using his jab and a straight right hand behind it to pound out a lopsided victory over former “Contender’’ series 1 reality TV star Sergio Mora.

Mora had surprised the boxing world three months ago when as a 6-1 underdog he won a majority decision and the WBC super welterweight title from an overconfident and over trained Forrest.

Forrest had spent three months in training because of postponements and rather than hone his skills it dulled them and left him six pounds over the 154-pound weight limit the day of the weigh-in. Although he got down he was drained and fought like it, winning the first three rounds easily and then seeming to hit a wall – and seldom Mora – after that.

This time was different. It was the 28-year-old champion who mysteriously came in two pounds over the weight while Forrest was both fit and sharp. He made that clear from the outset, never letting Mora into the fight by controlling the distance with a sharp jab that was an offensive weapon not merely a range finder and consistent combinations that left Mora bleeding and bewildered.

“My better was better than his better,’’ Forrest said. “I had a bad night in the first fight but you’ll didn’t want to believe it. Tonight it was Boxing 101 –  jab, jab, jab.’’

It was a bit more than that. Forrest never gave Mora room to breath or a distance to punch effectively. He smothered him on the inside, beat him with solid body shots and stuck him with the jab all night long.

For much of the second half of the fight Mora seemed crest-fallen. Normally cocky to the point of unwarranted arrogance, Mora’s body language was bad and he often seemed distracted in the corner when his handlers tried to instruct him. He had admitted before the fight that being in the position he now found himself in – being the hunted rather than the hunter – was “stressful.’’

“What is it they say, ‘Uneasy lies the head that wears the crown’?’’ Mora said. “I’m in a different position and that’s what produces the stress.’’

That and Vernon Forrest, who put Mora back into the position of being once again viewed as a creation of a reality television show rather than a legitimate world-class fighter. That might be unfair but no more so than the leg up on other young fighters like himself that the TV series afforded Mora.

“My legs weren’t there and my weight didn’t come off the way it usually does,’’ Mora (21-1-1, 5 KO) said after absorbing the worst beating of his boxing career. “I only had six weeks to train and I usually have 10. It was a short camp and I worked too hard. This guy is old. I should have won.’’

Vernon Forrest may be old but Sergio Mora could have had a 10-month camp and he wasn’t going to beat him Saturday night because this time the real Vernon Forrest was standing in front of him and this time punching back.

As for Joel Casamayor, he was old too but that had nothing to do what happened to him either. Juan Manuel Marquez was what happened to him.