A 2-2 draw at FC Dallas was enough to see Portland Timbers through to their ever MLS Cup final.
Second-half goals from Timbers pair Fanendo Adi and injury-time substitute Lucas Melano helped the Timbers to a 5-3 victory on aggregate on Sunday.
Ryan Hollingshead and Blas Perez had put Dallas 2-1 up on the night to give them hopes of a comeback.
After losing the first leg of their Western Conference matchup 3-1 in Portland last week, Dallas faced an uphill battle upon their return to Toyota Stadium.
And their hopes of advancing to the decider were dealt a major blow nine minutes into the second half, when Adi increased Portland's aggregate lead on the road.
The United States international outmuscled Walker Zimmerman and fired the ball past goalkeeper Jesse Gonzalez.
Dallas, though, refused to surrender as defender Hollingshead got on the end of Mauro Diaz's cross and guided a volley beyond Adam Larsen Kwarasey in the 69th minute.
The home side struck again four minutes later, taking a 2-1 lead on the night thanks to substitute Perez's header.
However, Portland dashed Dallas' faint hopes in the 95th minute after Melano sensationally dribbled the ball into the back of the net.
Standing in the way of the Timbers and the MLS Cup trophy on December 6 are Columbus Crew.
The Crew were beaten 1-0 at New York Red Bulls but still qualified 2-1 on aggregate in the Eastern Conference.
Anatole Abang came off the bench and scored for the Red Bulls in the third minute of stoppage time.
The Red Bulls almost levelled the tie two minutes later after Bradley Wright-Phillips' header rattled the crossbar as Crew held on in a thrilling finish.
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