The Sabres excellent effort coincided with Washington not being very sharp at all, and that was a brutal combination for fans of the Caps.
Playing the most important game of their regular season, the Capitals threw 45 shots at Ryan Miller but surrendered one odd-man rush after another in a 5-1 loss at the Verizon Center.
Capitals center Nicklas Backstrom has passed the final baseline test and will decide on his return.
The winner of tonight's game between the Caps and Sabres will gain sole possession of the eighth and final playoff spot.
Capitals captain Alex Ovechkin says his body is ready to shoulder the load down the stretch.