"I thought we were ready," goalkeeper Nick Rimando said. "I thought we played well, but set pieces can decide a game. And ... that's what happened. They caught us on that corner kick. We had some chances toward the end, but now we have to turn our focus to the next game."
That comes at San Jose on Saturday, when RSL will meet its closest pursuer in the MLS Western Conference.
The Earthquakes are 4-1-1 for 13 points just two behind RSL, which could easily be 7-0-0 instead of 5-2-0.
But the team that's usually rock-solid on defense has lost twice now because of lapses in the back.
Its 1-0 home loss to last-place Chivas USA last month came when Rimando lost his grip on a cross when he collided with defender Chris Schuler, allowing Chivas' Casey Townsend to poach an easy goal.
Against unblemished Sporting KC on Saturday, forward Alvaro Saborio lost his mark on defender Aurelien Collin in the 63rd minute, allowing Collin to head home a corner kick from Graham Zusi the 10th of the game for Sporting and snap the RSL defense's scoreless streak at 267 minutes.
"It is disappointing," coach Jason Kreis said. "It's something that we really focus on and we know that Kansas City is very, very good on all dead-ball situations. We talked about it a lot, we trained it this week. We don't typically give up goals on dead balls, either. We're pretty good at defending them."
League-leading scorer Chris Wondolowski and the Earthquakes spent the weekend earning a 2-2 draw at high-flying New York Wondolowski has seven goals, tied with New York's Thierry Henry and Kenny Cooper and will try to avoid a third straight home loss to RSL and snap a five-game winless streak against it.
RSL beat the Quakes on the road to open each of the past two seasons.
After that, RSL travels to FC Dallas, where it has never won.
In fact, it earned its first point at Pizza Hut Park with a scoreless draw last season, but still has just a 0-7-1 record there and has not scored there in its past three visits, since 2008.
"Salt Lake's a really good team," Sporting's Matt Besler said. "Their style of play is tough to play against. They have a lot of dangerous players who are capable of making plays. In a big game … it was nice that we were playing at home. Home field is a big deal in this league."
But still …
"They're a good, solid team with good possession on the ball," Sporting goalkeeper Jimmy Nielsen said. "They should be one of the favorites to win the MLS Cup this year."
Salt Lake at San Jose
P Saturday, 8:30 p.m.
TV • Channel 4