BUFFALO, N.Y. -- Casey Mittelstadt and Kieffer Bellows each scored two goals, Joseph Woll made 17 saves, and the United States began its title defense at the world junior hockey championship with a 9-0 victory over Denmark on Tuesday night.
In other preliminary round games, the Czech Republic upset Russia 5-4, Sweden beat Belarus 6-1, and Canada held on for a 4-2 win over Finland.
Max Jones, Kailer Yamamoto, Patrick Harper, Andrew Peeke and Dylan Samberg also scored for the U.S., which had three goals in the first nine minutes.
Mittelstadt, the most recent first-round draft choice for the Buffalo Sabres, scored his second goal to make it 5-0 late in the first period.
U.S. coach Bob Motzko remained undefeated in the world junior championships.
After winning last year's tournament in Montreal, the U.S. is seeking back-to-back gold medals for the first time. Denmark placed fifth last year, its highest finish in history.
In the most competitive game of the day, Filip Zadina and Filip Chytil scored second-period goals for the Czech Republic in a stunning 5-4 victory over Russia in the opening game.
Russia has medaled at the past seven world juniors, while the Czech Republic has not reached the medal round since 2005. This was the Czech Republic's second victory in its past 12 meetings with Russia.
Martin Necas, Filip Kral and Ostap Safin also scored for the Czech Republic, and Martin Kaut had three assists. Josef Korenar made 34 saves.
Zadina and Chytil scored to give the Czech Republic a 4-2 lead late in the second period. Filip Kraul scored the Czech Republic's fifth goal 6:16 into the third period.
Artur Kayumov and Vladislav Syomin scored late in the third period for Russia. Marsel Sholokhov and Alexei Polodyan scored in the first period.
Also Tuesday, Boris Katchouk had a goal and an assist as Canada beat Finland 4-2.
Katchouk's linemate Taylor Raddysh scored and added an assist for Canada, while Drake Batherson had the eventual winner, and Sam Steel scored a goal.
Carter Hart made 29 saves for the win.
Canadian defenseman Cal Foote made a heads-up play late in the third period, when he dove behind his goalie, swatting the puck away just as it touched the goal line and bouncing it off the post and out.
A video review at the next stoppage of play upheld the no-goal call on the ice.
"I wasn't sure I had it. The puck was on its side," Foote said. "But looking back at the replay, it was close, but I was pretty sure I got it."
It was the second video review that went Canada's way in the game. Katchouk's opening score was also upheld after Finnish head coach Jussi Ahokas challenged it for goalie interference.
Aleksi Heponiemi and Henri Jokiharju replied for Finland, while Ukko-Pekka Luukkonen stopped 28 of 32 shots.
In another preliminary round game, Lias Andersson scored twice for Sweden in a 6-1 win over Belarus.
Elias Pettersson and Erik Brannstrom each added a goal and an assist. Glenn Gustafsson and Jens Boqvist scored Sweden's other two goals. Alexander Nylander, Rasmus Dahlin and Fredrik Karlstrom had two assists apiece.
Yegor Sharangovich scored for Belarus. Andrei Grishenko stopped 31 shots.