1236 GMT March 30, 2020
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, AFP reported.
"The first round I was just looking, because I know he is a good fighter," the 36-year-old Golovkin said. "The second round it was true business."
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.
Alvarez will be free to fight once his suspension ends in August.
Saturday's event was moved from Las Vegas to south central Los Angeles and Golovkin fought for a fraction of the money he would have received for a fight against Alvarez.
With the victory he now joins Bernard Hopkins for the division record of 20 successful title defenses. Golovkin (38-0-1, 34 KOs) is now in line to surpass Hopkins in the fall when he hopes to finally get that rematch with Alvarez.