Tavares scored 3:42 into the first period to give Toronto a 3-1 lead, and he scored an empty-net goal with 44 seconds remaining in the third period.
They were his third and fourth goals of his first preseason with the Maple Leafs; he scored twice in a 4-1 win against the Ottawa Senators at Kraft Hockeyville Canada in Lucan, Ontario, on Tuesday.
Andrew Oglevie gave Buffalo a 1-0 lead 30 seconds into the game, and Ron Hainsey tied it 1-1 18 seconds later. Pierre Engvall gave Toronto a 2-1 lead at 1:11.
C.J. Smith got Buffalo within 3-2 at 11:28 of the first, and Tage Thompson tied it 3-3 6:08 into the second period.
Chris Mueller put the Maple Leafs ahead 4-3 at 10:55.
Curtis McElhinney saved all 20 shots he faced in the final 29:10 for Toronto, after Garret Sparks allowed three goals on 12 shots.
Linus Ullmark made 27 saves for Buffalo.
