DC United and Seattle Sounders both recorded 1-0 victories on Thursday night to advance to the MLS Conference finals.
Nick DeLeon scored in the 88th minute to lead United to a plucky 1-0 victory over New York Red Bulls, giving them a 2-1 aggregate win in the two-leg semi-final series.
A dominant but wasteful New York enjoyed the majority of possession for most of the tense clash at Red Bull Arena.
And the odds were further stacked against DC United when their starting goalkeeper Bill Hamid was sent off in the 69th minute after bringing down Kenny Cooper in the penalty area.
Backup goalkeeper Joe Willis was brought into the action, and Cooper stepped up to take the penalty.
He converted the shot but the goal was called off for encroachment after team-mate Thierry Henry entered the box before Cooper took the shot.
Cooper took the penalty a second time only to have Willis pull off a brilliant save.
Sensing their luck had changed for the better, United lifted.
In the 88th minute, late inclusion Robbie Russell set up DeLeon on the right side of the penalty area and the youngster swiftly put the ball past stunned Red Bulls shot-stopper Luis Robles.
The win puts DC into the Eastern Conference final, where they will play Houston Dynamo on Sunday.
Elsewhere, Mario Martinez scored with nine minutes remaining as Seattle Sounders beat Real Salt Lake at Rio Tinto Stadium to win their Western Conference semi-final 1-0 on aggregate.
Sounders advanced to the conference final for the first time and will visit the defending MLS Cup champions Los Angeles Galaxy on Sunday for the right to advance to MLS Cup 2012.