Ittawa could have repeated as division winners, But the Leafs have had a history of frustrating the Senators, eliminating them from the playoffs in three straight seasons from 2000-02.
Nieuwendyk helped add to the Senators' frustrations by scoring twice in a four-goal second period that gave the Leafs a 5-0 lead.
Ed Belfour made 28 saves for his second straight shutout and 10th of the season for the Leafs, who completed their regular season with 103 points, one more than Ottawa and Boston.
Toronto would win the division if Boston fails to win at New Jersey on Sunday.
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