In the first legs of their quarter-final ties, both MLS outfits were 1-0 victors to set themselves up nicely ahead of trips to Mexico.
Kansas City edged Liga MX leaders Cruz Azul, while the Galaxy held on for victory against Tijuana.
For Peter Vermes' Kansas City side, the clash at Sporting Park was the second of five games in 15 days.
But Kevin Ellis' 17th-minute goal was all they needed to take a huge step towards the semi-finals.
Picked out by Claudio Bieler, the full-back cut inside from the left before drilling a low shot inside Jose Corona's near post.
The hosts had the better of the chances during the first half and Bieler almost doubled their advantage, only to see his shot from just outside the area tipped around the post by Corona.
Mariano Pavone almost levelled for Cruz Azul during the second half, but after rounding Eric Kronberg, the Argentine saw his shot cleared off the line by a desperate Ike Opara.
Benny Feilhaber was fortunate to not see red for Sporting when he kept out a header with his arm on the goal line.
At the other end, Graham Zusi rattled the crossbar with a thunderous strike, but Sporting somehow held on.
Sporting will take the one-goal advantage to the Estadio Azul for the second leg on March 19.
At the Stubhub Center in California, Robbie Keane fortuitously set up Samuel on 11 minutes as the Galaxy overcame Tijuana.
The former Tottenham star was involved in the lead up but his failure to control a Marcelo Sarvas cross inside the area was capitalised on by Samuel, who was on hand to tap in the game's only goal.
