The Maple Leafs have won six straight, and a win over the Bruins would give them their longest run since winning 10 straight during the 1993-94 season.
One of the top catalysts for the Leafs' streak has been Sundin, who is in his best stretch of the season. The Toronto captain has scored a goal in six of the last nine games, including three game-winners in the last five.
"Mats is a guy that has scored very important goals for this hockey team," Toronto coach Pat Quinn said. "To me the guys that can score when there's pressure on, when there's not much room out there and in tight games, are a cut above."
On Tuesday, Sundin scored his ninth goal of the season while Joe Nieuwendyk had a goal and two assists to lead Toronto to a 5-4 win over the New York Rangers in the fight-filled second game of home-and-home series.
Owen Nolan scored the go-ahead goal in the second period, his fourth score in six games.
The Bruins, one point behind first-place Toronto, ended their four-game winless streak (0-2-1-1) with a 6-4 win over the Atlanta Thrashers on Wednesday.
Boston played penalty-free hockey for the first time in more than 21/2 years and Martin Lapointe scored the go-ahead goal in his 700th NHL game.
Joe Thornton had a goal and an assist for the Bruins, who weren't called for a penalty for the first time since March 28, 2001, against the Maple Leafs, who didn't have a penalty either.
"We had to win, no matter what," goalie Andrew Raycroft said. "We let it go and tried to focus."