Saborio scored in both halves and Ned Grabavoy added a goal in the 88th minute to give Salt Lake a commanding position going into Wednesday's second leg.
Saborio's first goal came in the 41st minute as he tapped the ball into an empty net after taking a centering pass from Chris Wingert. The goal was allowed despite a vociferous argument from Seattle that Saborio was offside.
His second tally came in the 53rd on back-heeled flick off a low cross from Javier Morales.
The Sounders will host the second leg at CenturyLink Field, where they went 9-4-4 record during the regular season.