But Knuble's wrist shot in the ensuing overtime enabled the Southeast Division-leading Caps to overcome the lapse and record their eighth straight win.
Boyd Gordon, Jason Arnott and Brooks Laich all scored and Braden Holtby turned aside 27-of-30 shots for the surging Capitals, who moved to within one point of Philadelphia for the top seed in the Eastern Conference.
The Blackhawks, meanwhile, earned a point in the Western Conference's tightly packed playoff picture.
Nick Leddy and Tomas Kopecky found the net and Corey Crawford made 38 saves for Chicago.
Celebrity Families of 2014 [PHOTOS]