A fortunate bounce and some solid defensive work helped LA win a first-leg edge over Tijuana.
The win marked the third of three positive results for MLS sides against Liga MX competition this week in the region's top club tournament, following Sporting Kansas City's 1-0 win over Cruz Azul and San Jose's late equalizer to steal a 1-1 draw against Toluca.
The Galaxy opened the scoring in just the 11th minute. Marcelo's long ball found Robbie Keane running into the box, and while the Irishman's first touch was poor, the ball fell fortuitously for the onrushing Samuel, and the on-loan Brazilian had a simple job to finish unchallenged from a few yards out.
The Galaxy held on for the rest of the match to take all three points, with goalkeeper Jaime Penedo especially impressive for the Californian side. At his most impressive, he atoned for his own mistake in the 61st minute, scrambling a ball off the line after he pushed a cross into Omar Gonzalez that deflected towards goal.
A few minutes later, Marcelo made his contribution to defense, standing his ground on the post and clearing a headed shot off the line.
The Galaxy will head south for the second leg of the quarterfinal against the Xolos on March 18.