Goals from Albert Rusnak, Jordan Morris and captain Nicolas Lodeiro powered the Sounders to a deserved win that leaves the membership getting ready to a first appearance in the final of the regional competition for teams from North America, Central America and the Caribbean.
Seattle took the lead after 16 mins with a beautifully crafted passing circulate that culminated in US international Morris shedding into the route of Slovakia international Rusnak, who rammed a low finish beyond City goalkeeper Sean Johnson.
New York were given lower back on level phrases within the 27th minute while a counter-attack ended with Thiago Andrade bundling the ball into the net from close range after an help from Argentine ahead Valentin Castellanos.
Seattle regained the lead seven minutes later with another slick move, Cristian Roldan cutting again from the byline for Morris to bludgeon a effective proper-foot shot into the pinnacle corner from just inside the place.
Seattle delivered a 3rd aim on 67 mins when the video assistant referee adjudged that New York defender Thiago Martins had fouled Peruvian striker Raul Ruidiaz within the region.
Mexican referee Luis Enrique Santander pointed to the spot and Lodeiro stepped as much as make it 3-1.
The second leg is in New York next Wednesday.
The winner of the tie will face the winner of the all-Mexican semi-final between Cruz Azul and Pumas UNAM. Pumas gained the first leg, at domestic, 2-1 on Tuesday.
