SEATTLE, Wash. - Portland Timbers forward Dairon Asprilla started just once in the final eight games of the 2018 MLS regular season after struggling to score in his opportunities on the field during the middle of the year. Asprilla finished the regular season with just one penalty kick goal.

But Asprilla came through with everything on the line Thursday night.

The Colombian forward scored in extra time before netting the game-winning penalty kick to lead the Timbers past the Seattle Sounders in a wild Western Conference semifinal series.

"Today, we demonstrated that we can do big things," said Asprilla, through an interpreter. "But we haven't won anything yet. Now, we have to be humble because there is still so much more ahead of us."

The Timbers entered Thursday's game with a 2-1 lead after winning at home in the first leg of the series. The Sounders beat the Timbers 2-1 in regulation in front of 39,542 fans at CenturyLink Field Thursday to tie the aggregate score up at 3-3 and send the game to extra time. After the two teams swapped goals in extra time, the Timbers won the series 4-2 on penalty kicks to advance to the Western Conference Championship series.

Portland will face either Sporting Kansas City or Real Salt Lake in the Western Conference Championship series. The series will begin on Nov. 25.

"Seattle and Portland brought the right rivalry to this match and it was incredible," Timbers coach Giovanni Savarese said. "There were moments when they scored when it looked like it would be difficult for us. Then we went back and forth, back and forth, all the way to PKs. I think it delivered a very good match."

Needing only a tie to assure themselves a spot in the Western Conference Championship after earning a win in their home leg of the Western Conference semifinals, the Timbers were happy to sit back and absorb pressure in the first half of Thursday's match.

The Sounders controlled 68.1 percent of possession and forced the Timbers to make 20 clearances in just the first 45 minutes. Portland didn't even take its first shot on target in the game until the 60th minute when forward Jeremy Ebobisse fired a shot straight to Seattle goalkeeper Stefan Frei.

After struggling to create dangerous chances in the first half, the Sounders came out ready to push for a goal in the second half.

Seattle finally broke through in the unlikeliest of ways. Timbers goalkeeper Jeff Attinella dropped a cross from Sounders midfielder Victor Rodriguez in the 68th minute, enabling Seattle forward Raul Ruidiaz to score easily. With a 1-0 lead, the Sounders needed only to prevent the Timbers from scoring to close out the series due to the away goal rule. If a series is tied on aggregate at the end of regulation, the club with more away goals advances, according to MLS Cup Playoff rules.

"Obviously, you want to do everything you can to help the team and when you make a mistake like that, it hurts," Attinella said. "But being a goalie, you got to try to shake it off because you always know there's another one coming."

With time winding down and the Timbers desperately needing a goal, Timbers midfielder Sebastian Blanco put Portland back in the game.

Blanco got on the end of a pass from Asprilla in the 78th minute and fired a shot from the top of the box into the corner of the net to tie the game up at 1-1 and give Portland a 3-2 lead on aggregate in the series.

"We believe that we can achieve great things being together," Savarese said. "The guys fought the entire match against a very good team. A team that wanted to give us everything that they had today. I'm very proud of the performance of our team."

The Sounders responded on a mistake from Blanco in second half stoppage time. Blanco accidentally headed the ball up to Ruidiaz while trying to make a clearance in the box, allowing the Peruvian forward to score on a volley to give Seattle a 2-1 lead in the game and tied the series up 3-3 on aggregate.

With the series tied on aggregate and both teams holding one away goal, the game went to extra time.

It was Asprilla's moment to shine. Asprilla, who entered the game as a substitute in the 72nd minute, headed in a long cross from Diego Valeri in the 93rd minute to give Portland a 4-3 lead on aggregate.

But Seattle tied the series back up four minutes later. Referee Jair Marrufo awarded the Sounders a penalty kick after Blanco committed a handball in the box following a Seattle corner kick. Sounders midfielder Nicolas Lodeiro sent his shot into the corner of the net to tie the series up at 4-4 on aggregate.

"I had two big mistakes," Blanco said. "But this is soccer. There's adversity. You need to recover really quickly and after that I went to the penalties (and I scored). We won and every teammate helped."

When Marrufo blew the final whistle to signify the end of extra time, Blanco and a few other Timbers began to celebrate. The Timbers insisted after the game that they were celebrating the chance to go to penalty kicks, but it looked on the field as if the players thought they had won due to the away goal tiebreaker, which doesn't apply in extra time. Whatever the Timbers thought in that moment didn't end up mattering.

Portland went on to beat the Sounders 4-2 on penalty kicks. Blanco and Attinella, who both made mistakes that led to goals from Seattle during the game, came up big during the penalty kicks. Blanco scored his penalty and Attinella made a save during the shootout.

Asprilla then closed the series out by firing the final penalty kick into the net.

"It was a very special moment," Asprilla said. "Thank God, we accomplished everything we wanted to tonight."

-- Jamie Goldberg | jgoldberg@oregonian.com

503-853-3761 | @jamiebgoldberg