Two quickfire goals in 11 first-half minutes gave them the perfect start at the City of Coventry Stadium, but they were made to work hard to claim the win as the Canadians hit back after the break.
After a bright opening, Japan opened the scoring after 33 minutes when the lively Nahomi Kawasumi put them ahead.
And a minute before the interval that lead was doubled - Aya Miyama beating Canada goalkeeper Erin McLeod.
The Canadians can take encouragement from their performance, and they restored their hopes with a strike from Melissa Tancredi after 55 minutes, but they could not find an equaliser.
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