When IBF welterweight champion Kell Brook challenged Gennady Golovkin for his middleweight titles last September, he was tied on two cards and leading by two points on the other when his corner threw in the towel to stop the beating Golovkin gave him in the fifth round. It was the 23rd straight stoppage by “GGG,” but the fact Brook landed early seemed like evidence of a chink in Golovkin’s armor.

WBA middleweight champion Daniel Jacobs (32-1, 29 KOs), who faces Golovkin (36-0, 33 KOs) in an HBO pay-per-view bout Saturday night at Madison Square Garden, respects GGG’s masterful skills but also believes his power in both hands might further expose that vulnerability.

“Brook shook him up,” Jacobs said earlier this week. “I don’t know if you could say Gennady was hurt, but he shook him up. For a welterweight to shake you up . . . that’s why I’m excited because, if I land those same shots, it might have a different effect.

“But he has really great posture where he keeps his head down and keeps his chin tucked. So, you’re going to have to get up under there if you want to test that chin.”

Jacobs displayed his power when he stopped Peter Quillin just 1:25 into the opening round of their bout 15 months ago. The Golovkin camp also expects Jacobs to be the heavier man at fight time, which makes him far more lethal than Brook.

“With Daniel it’s a more serious fight,” Golovkin said. “I know Daniel’s power. After the first round, I feel Kell’s power. A little bit different.”

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Trainer Abel Sanchez said GGG essentially lost respect for Brook after he got caught cleanly early in the bout and was unfazed. “In the corner, we were trying to get him to quit looking for the (KO) shot and just work for it,” Sanchez said. “Before the fight, he said he wanted to make it a bar fight or a street fight. That’s exactly what he made it once he hurt him because he had no respect for him. It didn’t thrill us in the corner, but it gave the fans a thrill.”

Jacobs is counting on Golovkin’s taste for mixing it up to expose what he believes are defensive flaws he’s seen in past fights. “We don’t know how he’s going to take my punches,” Jacobs said. “We don’t know how he’s going to take my speed. I want that opportunity like he gives everyone else for that one shot where he just has his head up — ‘C’mon hit me.’ I want that shot.”

If Jacobs comes to fight, Golovkin’s trainer Sanchez said, it could be a short, violent night. Given that Jacobs was knocked out in his one loss nearly seven years ago to Dmitri Pirog and that he was floored nearly two years ago in his first bout with Sergio Mora, which he won when Mora injured his right ankle, there are questions about Jacobs’ chin.

“Pirog was five years ago, this is long time ago,” Golovkin said with a shrug. “I remember Daniel had a problem. Right now, he is much better. He is more experienced. Last fight with (Mora), Sergio threw a perfect shot. Sergio is not a big puncher, but it was a perfect left hook, and Daniel was not ready. I think everybody has a chance.”

That’s what makes this bout so intriguing. Jacobs is in against the great Golovkin with a legitimate puncher’s chance.