The Portland Timbers needed 120 minutes and penalties to beat Sporting Kansas City 7-6 in the shootout after extra time ended 2-2 at Providence Park in Portland, Oregon.
Goalkeeper Adam Kwarasey was the hero for the Timbers, making several saves in the shootout and knocking in the decisive goal from the spot against SKC's backup keeper, Jon Kempin.
After a goalless first half, Rodney Wallace got the hosts out in front early in the second half.
Kevin Ellis, who also had a big goal in SKC's win against the LA Galaxy to make it into the postseason, scored late in the second half to force extra time.
Krisztian Nemeth was on the end of a superb buildup by SKC, and he cut the ball back past his defender and curled one in from the left side of the goalmouth from an impossible angle for what looked like the match-winner.
With extra time winding down, Max Urruti gave Portland new hope as he put away a shot from close range to send the match to penalties.
Kwarasey was man of the match, as he converted from the spot and then denied counterpart Kempin's attempt to seal the victory.
Portland will face Vancouver in the Western Conference semifinals.
The Montreal Impact crushed Toronto FC 3-0 to advance to the Eastern Conference semifinals on Thursday night in Montreal.
Surprisingly, it was Toronto who nearly opened the scoring, but Michael Bradley's redirected header from close range went straight into Impact keeper Evan Bush's hands.
Patrice Bernier broke the deadlock in the 19th minute, when, on a counterattack, he received a perfectly weighted through-ball from Ignacio Piatti before slotting under a helpless Chris Konopka in the Toronto goal.
Piatti doubled the Montreal advantage in the 33rd, pouncing on a defensive error outside of the Toronto box before curling his shot into Konopka's far post.
Didier Drogba then got in on the act six minutes later when he slotted home a Bernier cross from eight yards out to cap a near-perfect first half and seal the comfortable win for the home side.
Montreal will host the Columbus Crew in a home-and-home tie in the Eastern Conference semifinals.
For a full list of the MLS Cup Playoffs schedule, click here.