Giovani dos Santos scored the only goal of the game as Robbie Keane's LA Galaxy beat Colorado Rapids in the Western Conference semi-final first leg last night.
Dos Santos headed home 10 minutes after the restart to take the advantage going into next week's second leg.
Former Republic of Ireland captain Keane only came on for the final eight minutes after coach Bruce Arena opted to start 6'3" target-man Alan Gordon up front instead.
Seattle Sounders put one foot in the Western final with a 3-0 blitzing of FC Dallas.
Three goals in eight second-half minutes did the damage for Sounders, with Nelson Valdez and Nicolas Lodeiro finding the scoresheet.
Montreal Impact inflicted a rare defeat on New York Red Bulls as they took a first-leg advantage in the MLS Eastern Conference play-off semi-final.
The Impact won 1-0 thanks to Matteo Mancosu's superb volley just after the hour as the Red Bulls, who finished with 10 men after Omer Damari was dismissed, lost for the first time in the MLS since July.
Toronto FC scored twice in the last six minutes to earn a 2-0 win over New York City FC in the other Eastern semi-final.
The game looked set for deadlock until Jozy Altidore scored in the 84th minute and then Tosaint Ricketts grabbed a vital second in time added on.