The Union proved too hot to handle on Wednesday, defeating the Fire, 3-1, in Chicago for their first road victory since July.
Goals by Jack McInerney, Gabriel Gómez, and Antoine Hoppenot were enough to stop the Fire, ending their six-game winning streak at home.
The Union made two changes to the lineup that lost in Columbus, 3-2, on Saturday. Striker McInerney came in for Josué Martinez, while Keon Daniel replaced Gabriel Farfan.
It wouldn' take long - seven minutes - for those changes to take effect on a rain-slicked surface.
Amobi Okugo picked out Danny Cruz, who then passed the ball to McInerney for a well-placed low drive by goalie Sean Johnson for a 1-0 lead.
The Fire threatened in the 19th minute after Michael Lahoud was penalized on the edge of the Union box. Daniel Paladini stepped up to beat the wall but not the post as his curling free kick clipped the top of Zac MacMath's righthand post with the goalkeeper at full stretch.
The Union responded and had a chance to double their lead 26 minutes in. Cruz hit a cross-shot from a narrow angle, which forced Johnson to beat the ball away for a Union corner.
