Dallas needed an 88th minute winner from substitute Eric Avila to beat defending MLS Cup champions Real Salt Lake 2-1 in front of 11,000 fans at Pizza Hut Park in Texas.
Avila had only be on the field for a minute when he collected the ball outside the penalty area, dribbled past two defenders before placing a shot from 15 yards out past Salt Lake goalkeeper Nick Rimando's outstretched arm.
Both teams finished with 10 men after Salt Lake's Javier Morales was dismissed in the 69th minute after picking up a second yellow card for a reckless foul, with Dallas' Atiba Harris shown a straight red six minutes later for elbowing Will Johnson.
Both will be suspended for the second leg on Saturday which will determine who goes through to the Western Conference final to play either David Beckham's Los Angeles Galaxy or the Seattle Sounders, who play their semi-final first leg on Sunday.
In California, Joel Lindpere hit the 55th minute winner for the Redbulls, who were again without injured marquee signing Thierry Henry, as they edged the San Jose Earthquakes 1-0 at a cloudy Buck Shaw Stadium in Santa Clara.
The Estonian midfielder hit a precision shot past San Jose keeper Jon Busch after the home side failed to clear the danger in their penalty area.
The Red Bulls' World Cup winning striker Henry is hoping to overcome a knee injury to play in the second leg on Thursday.
Should Henry's side advance they will play the winners of the other Eastern Conference semi-final where the Colorado Rapids are protecting a slender 1-0 first leg advantage at the Columbus Crew on Saturday.
