Mantha added three assists and Mathieu Joseph, Dylan Strome, Dante Fabbro and Sean Couturier contributed a goal each as Canada cruised through the Group A game in Kosice, outshooting their opponents 56-12.
Britain returned to the top division after a quarter of a century. Canada plays host Slovakia on Monday. In the Group B game in Bratislava, Nico Hischier scored a late winner for Switzerland before Simon Moser added an empty net goal to beat Latvia 3-1.
Earlier Sunday, Anton Lander scored a hat trick to put Sweden's title defense back on track after demolishing Italy 8-0. Coming off a 5-2 loss to the Czech Republic, Lander opened the scoring on a rebound in the first period and added two in the final period, including a short-handed one, in Group B.
Patric Hornqvist scored twice for the two-time defending champion Swedes who scored six in the third period. They outshot the Italians 58-13. Italy remains the only team yet to score a goal at the tournament after conceding 17 goals in two games.
In Group A, Matthias Plachta and Frederik Tiffels' goals lifted Germany to a 2-1 victory over Denmark. The United States rebounded from an upset loss to Slovakia in the opening game by routing France 7-1.
After losing 4-1 to the hosts, the U.S. was not ready for another upset. Alex DeBrincat scored twice and Frank Vatrano added one in less than two minutes to set the pace early in the opening period for the U.S. to take command of the Group A game in Kosice.
Colin White also had two while Patrick Kane, who captained the Americans to the bronze medal at last year's worlds in Denmark, scored his first goal in Slovakia and added an assist. Chris Kreider added one for the U.S. while Anthony Rech had a consolation goal for France.
The U.S. next faces Finland on Monday. In Group B, star-studded Russia recorded its second straight win after blanking Austria 5-0. Evgenii Dadonov led Russia with two goals for the second consecutive game and Nikita Kucherov had a goal and an assist. Alexander Georgiev made 15 saves for the shutout.
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