Nelson Valdez found the back of the net again while Nicolas Lodeiro bagged a double as the Sounders exploded for three goals in eight minutes to take a big advantage to Frisco.
After a scoreless first half, the Seattle Sounders took a huge advantage in a Western Conference semifinal with Supporters' Shield winners FC Dallas and head into the second leg with a three-goal advantage.
The Sounders scored in the 50th minute to break the dam erected by a FC Dallas team playing three center backs and three players who typically play as central midfielders. Nelson Valdez took advantage of an error by FCD goalkeeper Chris Seitz to head in a goal in the second consecutive playoff game after not scoring during the regular season.