North Division-leading Toronto has lost two straight and five of six to drop out of the top overall spot in the NHL.
Brady Tkachuk and Ryan Dzingel also scored for Ottawa, and Joey Daccord made 33 saves for his first NHL victory.
''What an incredible feeling,'' Daccord said. ''The effort the guys gave tonight was outstanding. Just a really special moment.''
Matt Murray was unable to start, opening the way for Daccord.
''A couple minutes before warm-ups, (Murray) came back to the locker room and (said) `Hey, I don't know if I can play, I'm going to try in warm-ups.''' Daccord said. ''And then a couple of minutes into warm-ups, I just was skating by the bench. One of the trainers just gave me the point and I was like, `All right, let's do it.'''
Zach Hyman scored twice and William Nylander added a goal for Toronto.
Michael Hutchinson was pulled from the Toronto goal after allowing two goals in a seven-second span early in the first period. Frederik Andersen made 26 saves in relief. Andersen allowed five goals on 32 shots in a home loss to Winnipeg on Saturday night.
''I think we just really had a poor start,'' Toronto star Auston Matthews said. ''They scored a couple early. And we were just chasing the game the rest of the time.''
Maple Leafs: Host Calgary on Friday and Saturday nights.
Senators: Host Vancouver on Monday and Wednesday nights.
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