Rossi double helps LAFC get revenge on MLS champions Seattle
Rossi scored an early penalty and then snaked through the defence for a second eight minutes from time in the battle between the two Western Conference heavyweights.
Latif Blessing in the 39th minute and Brian Rodriguez in the 89th also got on the scoresheet for LAFC at the tournament in Orlando, Florida.
It marked a measure of revenge for LAFC who lost to Seattle 3-1 in last year's Western Conference final.
Seattle went on to beat Toronto for the MLS championship.
LAFC were missing star striker Carlos Vela, who scored a league record 34 goals in 2019, but opted out of this competition for family health reasons.
Rossi was brought down by Xavier Arreaga in the 14th minute but got up to slot home the penalty.
He embarrassed Arreaga again in the 82nd minute when he knocked through his legs before firing a low shot past goalkeeper Stefan Frei and could have had more goals if he had capitalized on numerous chances.
Sounders' elimination comes one day after New York City sent Toronto FC packing with a 3-1 win in a rematch of last year's Eastern Conference semi-final.
Earlier, the San Jose Earthquakes continued their impressive tournament run by reaching the quarter-finals with a 5-2 defeat of Real Salt Lake.
The teams were locked 1-1 at half-time after Cristian Espinoza's 21st-minute opener for San Jose was pegged back a minute later by Douglas Martinez.
But Magnus Eriksson put San Jose in front with the first of two second-half penalties in the 49th minute.
Vako made it 3-1 before Salt Lake's Damir Kreilach got one back in the 75th minute.
Chris Wondolowski in the 86th minute and Eriksson then sealed the victory for San Jose who will next face either Columbus or Minnesota United on Saturday.
"The team is growing game after game, showing a very positive attitude, their unity, their sacrifice, but more than anything is how they play," San Jose coach Matias Almeyda said.
"The best thing for a coach is being able to get the max level out of each individual player because I know that will allow us to have growth."
The "MLS is Back" tournament is being staged at a single location in Orlando, Florida after a four-month shutdown because of the coronavirus pandemic.
The tournament champion will earn a place in the 2021 CONCACAF Champions League.