The Fire pressed early on and Matt Watson came close to opening the scoring as he latched onto a low cross into the box, but his shot went inches past the post.
Chicago were dominating the game and they should have taken the lead when Grant Ward crossed for the run of Quincy Amarikwa, but he hit a tame shot straight at Joe Bendik, with only the keeper to beat.
The Fire could have doubled their lead just before the break when Amarikwa released the run of Alex down the left, but his low shot was well saved by Bendik.
In the second half, Chicago were given the chance to press further ahead when Ward was brought down in the area by Bendik and a penalty was awarded.
Jeff Larentowicz stepped to take the spot kick, but saw his effort well saved by the Toronto keeper, who then sprang to his feet to produce a remarkable reaction stop to deny Amarikwa on the rebound.
The home side continued to threaten the Toronto goal as Nyassi drove at his man and found space for a shot on the edge of the area, but his effort drifted wide of the post.
Chicago's inability to find the second goal cost them the three points as Toronto found an equaliser in the 89th minute when De Rosario fired in his first goal of the season to snatch a draw for his side.