The Sounders surged to an early lead, converting a corner kick in just the third minute. Gonzalo Pineda's service went over almost everyone but fell to unmarked Jalil Anibaba on the far post, and he played it back across for Kenny Cooper to smack in from close range.
The lead didn't last long. Diego Chara pressured Pineda into a turnover in a dangerous position, and he topped that with a vicious strike from distance to equalize.
Diego Valeri picked up his first goal of the season in the 14th minute to put Portland into the lead, taking a short cross in the box with his back to goal, turning Anibaba and shooting high into the net.
A defensive lapse allowed Dempsey to level the game at 2-2. Given space, Obafemi Martins collected and fed the criminally unmarked Dempsey, who had just Andrew Weber in goal to beat.
A pair of hammer blows in the early second half gave Portland what looked likea comfortable advantage. Chara provided the first in the 55th minute. Carrying the ball out of midfield, the Seattle defense - and Anibaba in particular - stood off him as he made his way towards goal. Unchallenged, he put a second shot fo the game from distance into the net.
Max Urruti's first goal of the season provided a cushion just two minutes later. Catching Seattle on the counter, he was just ahead of Anibaba's rushed defense to curl a shot from the edge of the penalty area.
Seattle fought back late, and Dempsey was the star of the show. Sliding in on Martins' cross, the USA captain managed to turn his shot past Stefan Frei from the top center of the box to make it 4-3 in the 85th minute.
Two minutes later, he had his third, and Seattle had its point. Ben Zemanski fouled DeAndre Yedlin in the box, and Dempsey converted from the spot to unleash pandemonium in the stands.
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