Second half goals by Sebastian Blanco and Diego Valeri on the road against Sporting Kansas City Thursday in the decisive match of the Western Conference finals propelled the Timbers to a 3-2 win and their second MLS Cup appearance in the last four seasons.

The 2015 champs will now meet Atlanta United in the 2018 finale on Dec. 8 in front of more than 70,000 fans in Georgia’s capital. Atlanta eliminated the New York Red Bulls in the East finals earlier Thursday, 3-1 on aggregate over the two legs.

Heavy favorites to advance before the match, SKC appeared in full control early on at Sporting Park. It was no surprise when the 2013 MLS Cup winner took the lead before the first half was over through Hungarian forward Daniel Salloi:

However, Sunday’s scoreless draw in Portland in the first leg of the two-game set effectively rendered that already slim lead paper-thin. The hosts were all too aware that a single strike by the visitors against the run of play would be enough to eliminate them via the away-goals tiebreaker, and they defended like it to start the second half.

“As far as I’m concerned, we come out at halftime it’s 0-0,” SKC coach Peter Vermes told ESPN’s Sebastian Salazar at the break.

Then their worst-case scenario happened. Crafty Timbers attacker Sebastian Blanco danced past a pair of players and found enough space on the left to cut inside and fire an all-world shot over Sporting keeper Tim Melia to instantly change the complexion of the series:

Sporting and their fans understandably were still stunned when last year’s league MVP Diego Valeri added a second for Portland less than 10 minutes after his fellow Argentine’s golazo, and suddenly Kansas City was on the brink of elimination, needing to score twice to eliminate the Timbers:

Gerso Fernandes made things interesting when he pulled one back for the hosts in the 81st minute. But Sporting couldn’t find the winner they needed even with more than nine minutes of stoppage time added at the end of the match. With Vermes’ side desperately pressing forward as the seconds ticked away, Valeri broke free and put the outcome beyond doubt by making it 3-2 Portland in the 99th:

It’s worth pointing out that the Timbers will also be significant underdogs when they head to Atlanta to take on the Five Stripes with MLS Cup on the line in front of what promises to be a record-breaking crowd at Mercedes-Benz Stadium. With relentless midfielder Miguel Almiron and record goal-getter Josef Martinez spearheading their attack, big-spending United is a formidable opponent, especially playing at home. But after Portland’s resilient performance in Kansas City, one gets the sense that’s a role that Giovanni Savarese’s team will actually relish.

Doug McIntyre covers soccer for Yahoo Sports. Follow him on Twitter @ByDougMcIntyre.

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