The first half was wide open, with both sides attempting to get on the board early and coming up short.
Evans opened the scoring in the 28th minute to put the home side on top. A flowing move from Seattle ended with Leo Gonzalez floating in a good cross from the left. The ball wasn't dealt with well and the clearing header made it just to the top of the box where Evans was able to settle it off his chest and take it on the half volley into the far corner.
In the 53rd minute Seattle had a brilliant chance to double the lead.
Clint Dempsey broke free of Seattle's back line and rounded Colorado goalkeeper Clint Irwin just outside the box. Irwin had no choice but to foul right on the edge of his area. Marc Burch's ensuing free kick came back off the crossbar and Colorado managed to clear the danger with the only damage done a yellow card to Irwin for the initial foul.
The match would take a dramatic turn late on. Seattle goalkeeper Micahel Gspurning caught a ball outside the area and was shown a straight red in the aftermath of the foul, giving Colorado a man advantage and a dangerous free kick against backup goalkeeper Marcus Hahnemann.
Seattle managed to clear the danger first from the free kick and then from a corner as the team looked to close out the final five minutes of the game.
Despite being down a man, Seattle came up with the better chances down the stretch. First it was Dempsey getting on the end of a pass from Eddie Johnson and curling his shot high and wide of the far corner. Then Johnson managed to put the game on ice.
This time it was Dempsey slipping Johnson through. One cut past a defender set Johnson up on his left foot to bang home his club's second and make sure of the Sounders progressing to the Western Conference semifinals.
The win sets up a Cascadia playoff showdown between Seattle and Portland, with the first leg set for Saturday in Seattle.