Marcus Johansson scored his second goal of the game, 6 minutes, 31 seconds into overtime, and the top-seeded Caps edged the No.
Auston Matthews scored his fourth goal in four games to break a 0-0 tie 7:45 into the third period, roofing a shot in on Braden Holtby and proving once again there is no stage too big for the Leafs' superstar rookie.
The Leafs made it a point of focus to come out stronger on home ice after being blown out of the water early on last week in Games 3-4.
They did just that, outshooting Washington 14-10 in the opening period and sending the message that the favored Caps would have to earn their way to the second round, it wouldn't be handed to them by a nervous, young team fumbling away the moment.
Instead, just like the entire series to this point, the Capitals once again had their hands full with the Leafs' speed.