Adonis Stevenson of Montreal was to defend the World Boxing Council version of the light heavyweight title against Tony Bellew later Saturday.
After an uneventful opening round, Kovalev dropped 15th-ranked Sillakh with a right.
The Russian was jumping up and down in his corner waiting to get back as the referee gave Sillakh (21-2) an eight-count, then ran up and floored him with another right that ended the bout only 52 seconds into the round.
The hard-hitting Kovalev (23-0-1), a Russian based in Fort Lauderdale, Fla., is often mentioned as a future opponent for Stevenson.
"I'm ready to fight any champion," Kovalev said after his first defence of the belt he won with a four-round KO over Nathan Cleverly in August.
There was a spectacular IBF welterweight elimination bout that saw Laval, Que., based Ionut (JoJo) Dan take a split decison over Quebec City favourite Kevin Bizier despite being deducted a point.
With the win, Dan takes over the No. 2 ranking and moves closer to a shot at the title currently held by Devon Alexander.
It was a close bout between fighters who rely more on technique than power. Dan (32-2) looked to land a few more shots than Bizier (21-1), who appeared to have turned the tide when he hurt his Romanian-born opponent with a right late in the ninth.
Bizier was warned twice about throwing Dan to the floor after clinches late in the bout, but it was Dan who then got a point deducted for holding.
The first judge gave it to Bizier by 117-110, the next had it 116-111 for Dan and the third scored it 114-113 for Dan.
"I think the ref gave him the point too easily," said Dan. "I didn't do nothing.
"I think my experience helped me to win. I respect Bizier. He's a great fighter, but even one point down, I win."
A battle of knockout artists had the crowd on its feet as David Lemieux (31-2), a ranked middleweight from Laval, knocked hard-headed Colombian Jose Miguel Torres (27-6) down seven times before the scheduled 10-round bout was stopped by the referee 1:48 into the seventh.
Torres was floored twice in the second round, again in the fourth, and twice more in the sixth and seventh before he finally stayed down after a crushing right from Lemieux.
Welterweight Mikael Zewski (22-0) of Trois-Rivieres, Que. stopped 35-year-old Ryan Lee Davis (24-12-3) with three knockdowns in the third round, one on a liver shot, of their scheduled eight-round bout.
Montreal-based Russian light heavyweight Artur Beterbiev (3-0) stung William Ray Bailey (12-18) with a jab and finished him with a short right to the temple only 2:49 into their scheduled six-rounder. The 2009 world amateur champion has needed only six rounds to win his three pro fights.
The crowd roared as local 32-year-old heavyweight Eric Martel Bohoeli (10-3) knocked down Didier Bence (9-1) of Montreal twice en route to a stoppage at 1:44 of the second round of their eight rounder.
Local favourite Sebastien Bouchard (8-0) won all six rounds against slow, light-hitting Belgian middleweight Mohammed Sidi Slimani (4-2).