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Portland almost doubled their lead as Fanendo Adi fired at goal from the edge of the box, and his effort was only parried into the path of Maximiliano Urruti who tapped home, but the forward was deemed offside by the linesman.
In the second half, Alvas Powell teed up Urruti with a chipped ball over the top of the Dallas defence, but his strike from a tight angle was well saved by Seitz.
However, Urruti did get on the scoresheet to put his side ahead once more as he rose highest in the area to guide Dairon Asprilla's pinpoint cross past Seitz from close range.
Dallas pushed for an equaliser late in the game and came within inches as Blas Perez directed a header at goal, but he was denied by an important save by Adam Kwarasey, who tipped his effort onto the crossbar and over.
The stop proved to be decisive as the Timbers sealed their victory in the 87th minute when Adi broke away upfield and teed up Diego Chara to beat Seitz with a cool finish.