Nicolas Lodeiro was at the double as Seattle Sounders accounted for FC Dallas 3-0 in the first leg of their MLS Western Conference semi-final, while New York Red Bulls went down to a Matteo Mancosu stunner.
Dallas won the Supporters' Shield after claiming the most regular-season points as the Sounders scrapped through their elimination play-off.
But the Sounders put their highly-fancied opponents to the sword in Seattle on Sunday, thanks to Lodeiro and Nelson Valdez.
Valdez, who came off the bench to send Sporting Kansas City packing last time out, broke the deadlock five minutes into the second half.
And that sparked a flurry of goals as Uruguay international Lodeiro netted a brace within eight minutes.
Eastern Conference high-flyers the Red Bulls suffered a late 1-0 loss in Montreal.
The Red Bulls, who topped the east in the regular season, succumbed to Mancosu's stunning 61st-minute half-volley.
Omer Damari was sent off late in Montreal, with the Red Bulls midfielder now set to miss the return leg in New York.
Toronto FC and LA Galaxy also posted first-leg play-off wins as all four home teams triumphed on Sunday.
Jozy Altidore and Tosaint Ricketts scored during the final six minutes to see Toronto past New York City 2-0 in the Eastern Conference clash.
In the west, Giovani dos Santos was the hero as the Galaxy edged Colorado Rapids 1-0.