Latif Blessing added a goal before halftime for LAFC, which avenged a 3-1 defeat to Seattle in last year's Western Conference final. Los Angeles' Brian Rodriguez added his second goal of the tournament to seal the team's meeting with Orlando City in a quarterfinal on Friday. Forward Bradley Wright-Phillips left the game at halftime for Los Angeles due to an apparent injury.
Will Bruin scored his first goal in more than 15 months for the Sounders, who were perhaps fortunate not to lose by more on a night they were outshot 25-11 overall and 8-4 in efforts on target.
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With reigning MLS MVP Carlos Vela opting out of the tournament for LAFC, Rossi has picked up the slack to move two goals in front of Toronto FC's Ayo Akinola in the tournament Golden Boot race, with TFC already eliminated.
Blessing and Eduard Atuesta had already contributed narrow misses before Rossi put LAFC in front from the penalty spot in the 14th minute. Rossi won the opportunity by inducing Xavier Arreaga into a clumsy challenge, and he finished past Stefan Frei's dive with a shot inside the right post.
The lead was two goals 24 minutes later. Blessing got to a square ball from Wright-Phillips on left side of the penalty area, cut around Cristian Roldan and Handwalla Bwana, then unleashed a shot that redirected off Seattle's Shane O'Neill, leaving a wrong-footed Frei no chance.
The Sounders answered in the 75th minute through a pair of substitutes, when Joevin Jones got down the left on a counterattack and served an outswinging cross just in front of the 6-yard box. Bruin met it, using his shin to propel it into the net off the underside of the crossbar.
From there, the Sounders had several chances to level the match. Kenneth Vermeer saved Raul Ruidiaz's close-range volley at the near post in the 75th minute. In the 81st, Jones' cross was just fractions too far for Bruin.
A minute later, Rossi capitalized on another error from Arreaga error to score a second goal before Rodriguez added his insurance tally in the 89th minute.