Brandywine native Seth Mitchell, right, connects to the head of Chazz Witherspoon during the first round of their NABO heavyweight title fight in Atlantic City. (TIM SHAFFER/REUTERS)

Seth Mitchell’s rapid ascent in the heavyweight ranks continued on Saturday night with a victory over Chazz Witherspoon when the bout at Boardwalk Hall for the vacant North American Boxing Organization belt was stopped in the third round.

The undefeated Brandywine native had Witherspoon on the ropes and wobbling at 2 minutes 31 seconds of Round 3 before referee Randy Neumann separated the fighters, keeping Mitchell very much in the mix for a world heavyweight title shot against unified champion Wladimir Klitschko.

Mitchell (25-0-1, 19 knockouts) had to battle back after a first round in which Witherspoon stunned the 10-1 favorite with several combinations. Mitchell appeared close to going to the canvas, but a rousing second round placed the momentum squarely back in his corner.

Mitchell dropped Witherspoon with a left hook early in the round and continued attacking after his Philadelphia opponent got back to his feet. Landing one body blow after the other, Mitchell got inside virtually at will.

Then came the decisive third round, in which Mitchell had Witherspoon (30-3, 22 KOs) retreating from the bell and clearly overmatched, as evidenced by a 51-11 advantage in power shots in Rounds 2 and 3.

“I was working my right hand,” Mitchell said. “I knew he was coming to fight. I’m going to be honest, I was nervous for this fight because he has a lot of experience. In the heavyweight division it only takes one shot. I could see it in his eyes. He was coming for me, but I never stopped using my body shots. They were slowing him down, and he had no defense for them.”

Saturday’s co-main event to Chad Dawson’s decision over Bernard Hopkins was the first scheduled 12-rounder for Mitchell, who has grown accustomed to ending matters early. More than half of his fights have not lasted beyond two rounds, and he’s quick to point out the less punishment he takes, the longer his career figures to last.

This bout presented the most damage Mitchell has withstood in any of his recent fights, and it showed his fortitude after admitting during the post-fight news conference that he felt “the stinky leg” when Witherspoon connected in the early going.

“After the first round after Seth got hurt, I basically told him to grab [Witherspoon], tie him up and get his legs back together,” trainer Andre Hunter said. “I wasn’t worried about him trying to get it back so soon. I knew we had a long fight, and I told him sooner or later, [Witherspoon] was going to run into your shots.”

Mitchell’s last fight was Dec. 10 at the Walter E. Washington Convention Center, where he scored a second-round TKO of Timur Ibragimov in a performance that continued to advance his standing as legitimate contender for the unified heavyweight title. Ibragimov had not been knocked out before that fight, and Mitchell sent his Uzbeki opponent to the canvas with relative ease.

That hasty result was in line with Mitchell’s triumphs over the past two years. The All-Met football player from Gwynn Park won each of his past nine fights by knockout, and none lasted more than five rounds. His victory over Ibragimov followed a third-round TKO of Hector Ferreyro, a first-round knockout of Evans Quinn and a second round TKO against Charles Davis.

None of those opponents was nearly as highly regarded as Witherspoon, whose only losses came to rising heavyweight championship contenders Tony Thompson in December 2009 and Chris Arreola a year and a half earlier. Thompson, in fact, beat Witherspoon at Boardwalk Hall in his second fight back after losing to Klitschko in an 11-round knockout.

Mitchell has designs on Klitschko’s titles, including the International Boxing Federation and World Boxing Organization belts, and beating Witherspoon moves him that much closer to realizing his dream. Mitchell has spoken repeatedly of his ambition to fight Klitschko for the heavyweight championship of the world.

“I knew I had it in me,” Mitchell said of his victory over Witherspoon. “The dog hadn’t been brought out of me yet, but I’m built for that. If I get knocked down, if I can get up, I’m going to get up. Some fighters, they get knocked down and have some quit. I have no ounce of quit in me.”