It continued that way for nearly the entire fight. Hopkins (55-6-2) got stronger later, nailing Shumenov with a straight right to the jaw that made his opponent's knees buckle and sent him to the canvas in the 11th round for a mandatory eight-count.
Shumenov, 30, got in a few good shots, but had to relinquish his World Boxing Association and International Boxing Association title belts to Hopkins, who already had the International Boxing Federation championship.
Before the decision was announced, Hopkins yelled to the crowd, "I'm a 15-round fighter."
Hopkins was seeking to become the oldest boxer to unify world titles. He already holds the distinction as the oldest holder of a championship belt, having defeated Tavoris Cloud on March 9, 2013 for the IBF version of the title. He defended the belt last Oct. 26 with a unanimous decision over Karo Murat in Atlantic City.
Hopkins now will seek to make a total unification of the light-heavyweight title if he can secure a bout with Adonis Stevenson, the World Boxing Council light-heavyweight champion.
After trying to feel out his opponent in the opening rounds, Hopkins found that he could hit Shumenov (14-2) pretty much at will, and that Shumenov couldn't hit him with any regularity. As in his custom, Hopkins picked his spots but he nailed the Kazahkstan citizen with straight rights to the head.
In the fifth round, Hopkins scored big with a combination of a straight right to the temple and a left hook to the jaw. He followed up in the sixth round when he snapped Shumenov's head back with a left hook, his hardest shot of the fight to that point.
Shumenov got in a good shot or two in each round but he missed far more than he connected. Through six rounds, he had landed just 51 of 267 punches, an abysmal 19 percent mark.
Hopkins usually punctuated his round performance with some good punches just before the bell, such as a crushing left cross at the end of the eighth round, and a right-hand counter and straight left to the cheek as the finishing blows to the ninth.
In the 10th round, Hopkins decided he wanted to engage Shumenov on the ropes. It was a risky move but Hopkins managed to fight his way out without suffering too much damage. In the 11th round, in Shumenov's corner, Hopkins actually gestured for his opponent to come and get him.
In the co-main event, Shawn Porter retained his IBF welterweight championship with a fourth-round technical knockout over former champion Paulie Malignaggi. Earlier, Peter "Kid Chocolate" Quillin retained his World Boxing Organization middleweight title with a unanimous 12-round decision over Lukas Konecny.