FRISCO, Texas - Walker Zimmerman forced extra time with a 91st-minute goal and scored the clinching penalty kick to help top-seeded FC Dallas reach the MLS Cup Western Conference finals on Sunday night advancing past the Seattle Sounders in their two-game series.

FC Dallas won 4-2 on penalty kicks after three lead-changing goals in the final 10 minutes of regulation time and 30 scoreless minutes of extra time. The result after first 180 minutes of the two-leg series was a tie in both the aggregate score (3-3) and away goals (1-1).

FC Dallas will face the Portland Timbers in the conference finals, with the first leg in Portland on Nov. 22.

"It was a very emotional game," Dallas coach Oscar Pareja said. "We are very happy for the youngsters who stepped up today in a very difficult situation. Today was all about heart and our players did it."

The fourth-seeded Sounders, having won the first leg 2-1, tightened their defence to hold the series edge for the first 83 scoreless minutes before Tesho Akindele broke through in the 84th, heading home Je-Vaughn Watson's cross.

Pareja inserted Akindele and Zimmerman into the game only moments before the opening goal.

"When I was subbing on, I was thinking there was no way we were going to overtime," Akindele said. "When we scored, I thought we were going to win."

The 2-2 aggregate score after Akindele's goal would have been enough for FC Dallas with the away goals tiebreaker, but Akindele's goal set off a frantic finish that featured three goals in eight minutes. The Sounders tied the game and took back the series lead when Chad Marshall headed in Marco Pappa's corner in the 90th.

Less than a minute later, Zimmerman made it 2-1 for FC Dallas with a turnaround header which forced the two 15-minute, extra-time periods.

"They were throwing a lot of people forward and they had a lot of big guys in the box," Seattle coach Sigi Schmid said. "They were getting opportunities to hit crosses and we weren't putting enough pressure on the ball on the service so they were hitting crosses and they were all around the goal."

Dallas goalkeeper Jesse Gonzalez saved penalty kicks from Andreas Ivanschitz and Chad Barrett in the shootout while his teammates were perfect in getting past Seattle goalkeeper Stefan Frei. Mauro Diaz, Ryan Hollingshead and Akindele all scored before Zimmerman's clinching penalty and setting off Dallas' celebration.

"We know their tendencies and what they've done in the past, and you see if you feel like they're going to stick to them," Frei said. "If they look like they're nervous, or look like they're comfortable then that might change your take on it, but they switched up all their shots today. They finished strong and we didn't."

Dallas advanced to the Western Conference finals for the first time since losing to Colorado in the MLS Cup final in 2010.

Seattle was trying to reach the Western Conference finals for the second straight season and the MLS Cup remains the one trophy missing from its resume. The Sounders were content to play defence most of the night and try to escape with a 0-0 draw. Seattle was fortunate to hold on as long as it did thanks mostly to the play of Frei who made critical saves in the 52nd and 65th minute to keep the game scoreless. Frei finished with eight saves.

"Stefan, obviously, kept us in the game. Penalty kicks are penalty kicks, you can win or you lose," Schmid said.