Chicago's Chris Rolfe and Brian McBride scored in the second half as the visiting Fire pulled out a 2-0 win over Toronto FC on Saturday.
The loss snapped a four-game unbeaten streak for Toronto (3-3-4), which squandered a chance to move ahead of D.C. United for top spot in the Eastern Conference.
Instead, Chicago (3-0-6) leapfrogged both Toronto and D.C. by winning on the heels of a single-season record five consecutive draws to remain the lone undefeated club in Major League Soccer.
"Hopefully, we keep our hands up and keep improving," Toronto's Dwayne De Rosario told CBC Sports.
After a slow first half — the first shot didn't come until Amada Guevera's try in the 28th minute — Toronto dominated much of the action. But the Fire made the most of their opportunities.
Rolfe's goal in the 55th minute came moments after teammate Logan Pause got away with an apparent hand ball in his own penalty area.
With the Toronto crowd still voicing its displeasure over the non-call, Rolfe took Cuauhtemoc Blanco's feed through a seam in the middle of the defence, chipped the ball past onrushing goalkeeper Stefan Frei and tapped the bouncing sphere into an open net for his second goal of the season.
Toronto generated several glorious opportunities in the ensuing minutes — including a Guevera free kick that rang off the crossbar and some barely off-target headers by Chad Barrett and Pablo Vitti — but couldn't get any of its 11 shots on the day past Fire keeper Jon Busch.
De Rosario suggested his team's attack may have been stymied in part by the artificial surface on a soggy day at BMO Field.
"This turf's gotta go," he said. "I'm sick of it."
The Reds' inability to convert allowed McBride to put the game out of reach in the 81st minute. After Frei made a diving save to his right, Mike Banner pounced on the rebound and slipped a perfect pass over to McBride, who buried his sixth of the season.