Supporters' Shield champions the Sounders head into Monday's second leg at CenturyLink Field on level terms with Dallas after last week's 1-1 draw.
The Sounders were arguably the happier of the two teams thanks to Osvaldo Alonso's equaliser and away goal in Frisco, Texas.
Sigi Schmid's Sounders now return to familiar surroundings with a simple equation - win to secure a berth in the Western Conference finals.
"We still need to win at home. We definitely don't want to get a 2-2 tie at home, but having an away goal certainly helps," Schmid said after Sunday's draw.
"It means they [FC Dallas] know in order for them to come out with anything in Seattle they've got to win the game, and if they're not going to win the game, they've got to get two goals.
"It puts a little more pressure on them, to a certain extent. But I know we'll play better being at home, and playing in front of our crowd is a good thing for us."
Mike Petke's decision to drop Australia international Tim Cahill for Peguy Luyindula proved to be a masterstroke, with the veteran Frenchman a constant threat throughout as the New York franchise won 2-0 at Red Bull Arena.
United, who finished top in the East by four points, face an uphill task to salvage their season but coach Ben Olsen has not given up hope upon Saturday's return leg in Washington.
"One thing I do believe in is that this team responds to adversity," Olsen told MLSsoccer.com.
"There's real character with these guys and I have no doubt that we will be a better side [than we were last weekend.] Like I said before, we'll make this thing very interesting."
"I think both teams will bring that same kind of physicality," said Heaps. "I don't think any team is going to back down."
