Argentine pair Ignacio Piatti and Victor Cabrera got on the scoresheet for the Impact at the Stade Olympique, as the home side took a significant step towards a maiden appearance in the Champions League final.
No Canadian club has ever played in the final of CONCACAF's premier club competition - including in its previous guise as the CONCACAF Champions Cup - while only Toronto FC have reached the Champions League semi-finals in its previous six editions.
Frank Klopas' Impact blitzed two-times champions Alajuelense early, with Piatti opening the scoring in the ninth minute.
Right-winger Dominic Oduro got behind the Costa Rican visitors' defence and smashed his cross towards the penalty spot where Cameron Porter missed an attempted flick, but Piatti controlled the ball at the back post, before beating his opponent and sending a scorching shot high into the net.
It was 2-0 in the 14th minute when a corner to the back post bounced off Montreal centre-back Bakary Soumare and Cabrera's snap shot bounced past a number of players to nestle in the net.
Alajuelense were eliminated from the Champions League at the same stage last season by eventual runners-up Toluca.
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