Clint Dempsey scored on his MLS return as the Seattle Sounders were beaten, while the LA Galaxy also went down.
Dempsey, making his first competitive appearance since August last year after heart problems, was on the scoresheet as the Sounders lost 2-1 at the Houston Dynamo.
His second-half goal at BBVA Compass Stadium came after the reigning champions had fallen behind on Saturday.
Erick Torres opened the scoring in the 20th minute as his 20-yard free-kick went over the wall and in.
Romell Quioto doubled the lead with his first MLS goal just before half-time, bending an effort from range past Stefan Frei.
The Sounders had hope thanks to Dempsey, who struck after a cross fell to him at the back post, but Seattle were unable to find a second.
The Galaxy also suffered defeat in their opening game, losing 2-1 at home to Dallas.
Maximiliano Urruti struck first with an effort into the bottom corner from the edge of the area in the 47th minute.
While Giovani dos Santos levelled from the penalty spot, Kellyn Acosta secured Dallas' victory with a neat finish from inside the area.
Elsewhere, a Dominique Badji goal saw the Colorado Rapids edge the New England Revolution 1-0.
Anibal Godoy's effort helped the San Jose Earthquakes beat a 10-man Montreal Impact 1-0.
DC United drew 0-0 at home to Sporting Kansas City, while Real Salt Lake's meeting with Toronto was also goalless.
The Columbus Crew and Chicago Fire played out a 1-1 draw.