Tie Domi and Joe Nieuwendyk scored third-period goals for the Maple Leafs, who lead the best-of-seven series 3-2 and can advance to the second round by beating Ottawa on Sunday.
"It's a big win for us, but the job is not done yet," said Toronto's Matt Stajan, who replaced injured captain Mats Sundin.
Sundin was out because of a left leg injury.
"You haven't seen the last of us yet. We're going to force this to Game 7 and come in here next Tuesday and win this series," Ottawa captain Daniel Alfredsson said.
The Senators outshot the Maple Leafs for the fifth straight game, but couldn't get the puck past Belfour.
He recorded his 14th career playoff shutout despite not having to make any spectacular saves.
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