The Sounders and the Galaxy are level on 60 points at the top of the division, with the Seattle club holding the advantage due to having won two more games than the California outfit.
With the duo holding eight more points than Eastern Conference leaders DC United with three games to play, the fight between the top two in the West looks set to decide who will claim the Supporters' Shield for the best record in the league.
That prize is likely to be decided in nail-biting fashion as the pair face each other home and away in the final two games of the season, making the last encounter before that effective play-off for the shield even more pivotal.
And both the Sounders and the Galaxy will need to be on top form to overcome teams with MLS Cup play-off ambitions of their own.
Seattle host a Whitecaps side two points outside of a play-off place on Saturday before, on Monday, the Galaxy take on a Dallas team that can secure a top-five spot with a win should the Whitecaps fail to emerge victorious.
Dropped points from the Whitecaps will also give Real Salt Lake the opportunity to clinch a play-off berth with victory over a San Jose Earthquakes team, whose only remaining goal is to avoid bottom spot in the conference.
Over in the Eastern Conference the picture is less clear, with DC United the only side absolutely guaranteed of a place in the post-season.
Seven teams from second to eighth in the East are in the fight for the remaining spots, although New England Revolution will be expected to seal one with an away win at bottom side Montreal Impact.
Sporting Kansas City will ensure a play-off place by beating Chicago Fire, providing that New York Red Bulls prevent Toronto from claiming all three points at the Red Bull Arena.
A triumph for the Red Bulls will see them into the knock-out rounds should Philadelphia Union and Houston Dynamo fail to overcome Columbus Crew and conference leaders DC respectively.
Elsewhere, Chivas USA host Colorado Rapids.
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