Extra-time goals from Kenny Cooper and Marco Pappa saw the Sounders beat Portland 3-1 in their US Open Cup quarter-final, after the Timbers had midfielder Diego Chara sent off in the 99th minute at Starfire Sports Complex.
The Timbers will return to Washington on Sunday to play Seattle for the second time in five days - this time in the MLS and at the Sounders' regular home ground, CenturyLink Field - and Caleb Porter's men will be looking for their first win in four matches in all competitions.
Chara will miss the match through suspension, due to an accumulation of yellow cards in the MLS.
Porter may be tempted to bring in new signing Liam Ridgewell after watching his defence again fail to keep a clean sheet on Wednesday.
Ridgewell, formerly of West Brom, signed for the Timbers late last month.
Portland have the equal-second-worst defensive record in the MLS this season, having conceded 30 goals in 18 matches to sit seventh in the Western Conference.
Seattle, who have won six of their past seven games in all competitions, are top in the West and could move 10 points clear should they beat Portland and other results go their way.
Cooper, the former Timbers striker, boasts four goals from his last five appearances in all competitions, but is likely to be named as a substitute on Sunday, with United States international Clint Dempsey in line to feature after his stint at the FIFA World Cup.
The nearest challengers for Seattle in the Western Conference are Real Salt Lake, who ended a poor run of form with a 2-1 win over New England Revolution last week.
With goalkeeper Nick Rimando and defensive midfielder Kyle Beckerman in Brazil, Real had not won since mid-May but broke a six-game winless streak against New England on July 4 as their key duo returned to action.
Real's next test is a trip to Los Angeles Galaxy, unbeaten in six league matches, on Saturday.
The Revolution lost a US Open Cup quarter-final 2-0 to Philadelphia Union on Tuesday, extending their losing run to three straight games in all competitions.
Philadelphia will host Colorado Rapids in the MLS on Saturday, while Chicago Fire - 3-1 winners over Atlanta Silverbacks in the US Open Cup - will aim for their first league win in seven matches when they visit New England.
San Jose Earthquakes, the West's bottom club, will start the weekend's action with a game against DC United on Friday, while Toronto FC host Houston Dynamo, Sporting Kansas City travel to Montreal Impact and Vancouver Whitecaps will take on Chivas USA.
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