The Whitecaps will return home with an impressive leg when the second leg of the series is played at Vancouver's BC Place next Wednesday.
The winner of this series will play either Toronto FC or the Montreal Impact in the championship final.
Hassli, who has yet to score in seven Major League Soccer appearances this year, put the Whitecaps up 1-0 in the 18th minute. Hassli was set up beautifully just outside the box by Omar Salgado's centring pass, and the striker kicked a shot past Edmonton goaltender David Monsalve as a sparse crowd at Commonwealth Stadium looked on.
Salgado was dangerous for much of the first half, and he helped put the Whitecaps up 2-0 by setting up Atiba Harris's goal in the 41st minute. Salgado got by an Edmonton defender in the box and fired an ambitious shot from a sharp angle that bounced off the right post. Harris was there for the easy tap-in to make it 2-0 Vancouver.
Edmonton's Serisay Barthelemy left the pitcher on a stretcher after taking a knock in the 21st minute. He was replaced by Matthew Lam.
Harris had to be replaced in the 71st minute by Michael Nanchoff after he picked up an injury.
Long Tan almost made it 3-0 in the 85th minute, but his long strike from just outside the box was parried by Monsalve.
Brad Knighton picked up the clean sheet for Vancouver.