United States internationals Jozy Altidore and Michael Bradley scored two priceless away goals inside the final 22 minutes in Toronto's drama-filled 3-2 defeat in Montreal on Wednesday (AEDT).
The first-leg fixture had it all in front of a capacity crowd at Stade Olympique with kick-off delayed in the all-Canadian affair after the referee determined the penalty area had been marked incorrectly.
When the lines were resprayed and the match finally got underway, 38 minutes later than anticipated, the 61,004-strong crowd were treated to a thriller as the Impact - with Didier Drogba named among the substitutes - raced out to a 2-0 lead.
Dominic Oduro opened the scoring in the 10th minute, tucking the ball underneath the onrushing Clint Irwin, before Matteo Mancosu doubled Montreal's advantage two minutes later.
It got better eight minutes into the second half, when Ambroise Oyongo made it 3-0 to put the Impact firmly in control.
But the visitor had other ideas, experienced duo Altidore and Bradley pulling goals back in a five-minute span - the first coming in the 68th minute - to leave the tie delicately poised ahead of the return meeting in Toronto.
In the Western Conference, Seattle Sounders came from behind to beat the visiting Colorado Rapids 2-1 in the first leg.
The Rapids took a 13th-minute lead via Kevin Doyle, but Seattle's Jordan Morris equalised six minutes later.
And Uruguay international Nicolas Lodeiro completed the comeback from the penalty spot just past the hour-mark in Seattle.