After a scoreless draw against Vancouver on Saturday at Rio Tinto Stadium, RSL finished the Major League Soccer regular season with 57 points.
Seattle trails by one, pending a final game against the L.A. Galaxy.
If the Sounders win or tie, they become the No. 2 seed in the West and the two-game aggregate series opens Saturday in Utah.
If the Sounders lose, Real Salt Lake is the No. 2 seed and the series opens Friday night at Seattle.
In both scenarios, Game 2 will be Nov. 8.
"It's going to be tough," said defender Chris Wingert. "There are a lot of teams in top form right now. It will be a huge challenge. But we're looking forward to it."
Injuries take their toll
RSL's starting lineup against Vancouver was missing some familiar faces.
Kyle Reynish started in goal for Nick Rimando, who was resting a sore shoulder.
Meanwhile, Javier Morales was sidelined because of the flu, Nat Borchers rested a quad injury and defender Jamison Olave continues to recover from a hamstring injury.
Coach Jason Kreis also rested midfielder Will Johnson, who did enter the game in the 79th minute.
The good news for RSL?
Midfielder Ned Grabavoy returned from a monthlong absence because of a quad injury. Defender Kwame Watson-Siriboe, who missed five games because of a leg strain, also returned.
RSL drew its seventh consecutive sell-out crowd (20,483). That's a club record. ... Real Salt Lake is 4-1-3 in its last eight Major League Soccer games. It has outscored the opposition, 9-3. ... After three straight games without a goal, Kreis joked, "We put them in the bank and decided to save them up for the first round of the playoffs."