The Russian, who ripped the belt from Nathan Cleverly in August, knocked his Ukrainian opponent down with a huge left early in the second.
He then proceeded to taunt the 28-year-old, who lay scrambled on the canvas before groggily getting to his feet, nose bloodied.
The referee delivered the eight count, but the contest was over moments later as Kovalev finished the job with two more thudding shots that sent Sillakh flying under the ropes.
It was a devastating display of power from the 30-year-old champion, who has been talked up as a possible opponent for WBC titlist Adonis Stevenson.
However, for that fight to happen, the Canadian must first overcome Liverpool's Tony Bellew in the headline bout.