IBF, IBO, WBA, WBC middleweight world champion Gennady Golovkin closed the show landing 11 of 15 power punches on Vanes Martirosya in round two after getting outlanded 6-5 in power shots in round one.
Golovkin equalled the record of 20 straight world middleweight title defences with a devastating second-round knockout of Martirosyan on Saturday.
Golovkin ended the fight with a vicious series of right and left combinations that floored the heavy underdog Martirosyan, who appeared doomed from the outset of the hastily arranged fight at the StubHub Center.
Martirosyan was in big trouble from the beginning of the second round as Golovkin rendered the challenger helpless by landing a right uppercut on the chin.
He followed with a few stinging jabs and then a full throttle left hook and straight right on the ropes that put Martirosyan down for good at 1:53 of the round.
The American tried to get up before the 10 count but slumped forward, ending up face down on the canvas. Golovkin, who is boxing's longest-reigning current world titleholder, is now unbeaten in 39 fights, including 34 by knockout.
Martirosyan was a last-minute replacement following the cancellation of the Kazakh's much anticipated rematch with Canelo Alvarez of Mexico.
The rematch was cancelled because of Alvarez's two failed drug tests in February, followed by his suspension by the Nevada State Athletic Commission on April 18.