Mauro Rosales put in a gutsy performance and netted the match winner as Seattle topped Philadelphia.
Seattle goalkeeper Bryan Meredith earned a clean sheet in his first MLS start, but it was a moment of brilliance from two of the Sounders' best that proved to be decisive.
After being held out for just over an hour by the ever-impressive Zac MacMath in the Union goal, Seattle found a winner when Montero played a brilliant ball across the top of the box to free Rosales on the right.
The veteran midfielder, who was a question coming into the match with an injury, had time to take a couple of touches and steady himself before firing a low shot into the net.
The Union never really threatened Seattle's goal late on and the Sounders really should have added to the lead when Montero curled a shot back off the bar.
Seattle will be in action again on Wednesday with a trip to Dallas while Philadelphia will have to wait until May 13 to host New York.
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