German pitcher Mike Bolsenbroek got the win, allowing one unearned run in 5 1/3 innings with six strikeouts. Max Kepler sparked the German offense with a triple, three runs and two RBIs, while leadoff hitter Eric Suttle got two hits and scored twice.
"A lot of planning, a lot of work and a lot of preparation went into this, and the team came out ready," said Germany manager Greg Frady. "It was very exciting to finally start, and we got the start we wanted."
The host Germans will take on Canada on Saturday after the Czech Republic plays Great Britain, the loser of which will be eliminated from the tournament.
Mike Cervenak and Martin Drong each had two of the Czech's five hits in the game, and Czech starter Michal Sobotka took the loss.
"Our game plan was to play as tough as we could," said Czech manager Andrew Berglund. "[The Germans] came out swinging from the first inning. They proved that they belong here."
Germany jumped on Sobotka right away in the top of the first inning with three runs. After Suttle opened the game with a double, Matt Vance was hit by a pitch. Kepler, a highly touted prospect in the Minnesota organization, tripled down the right-field line. He scored on a Donald Lutz groundout to second base for a 3-0 lead.
Sobotka settled down thereafter and retired 12 of the next 13 batters before hitting Kai Gronauer with a pitch in the fourth inning.
The Czechs were in scoring position for the first time in the third inning, with Drong on second and two outs. Drong advanced to third on Cervenak's roller to German third baseman Weaver, who did not throw to first but twirled around and tossed to shortstop Vance at third to catch Drong rounding the base too far.
The Czech Republic trimmed the deficit to 3-1 in the fifth inning with an unearned run. Jakub Sladek walked, stole second and was on third after a Drong hit. Sladek scored when Lutz could not hold onto Bolsenbroek's pick-off attempt at first base.
Germany answered in the bottom of the fifth with eight runs to blow the game wide open.
Toby Gardenhire started the onslaught with a double to left and scored on Vance's sacrifice bunt. Kepler followed with a grounder through the legs of first baseman Pavel Budsky, scoring another run before Lutz walked to load the bases. Petr Minarik came on to replace Sobotka and promptly gave up an RBI single to Aaron Altherr and walks to Simon Guhring and Gronauer -- the latter forcing in Lutz. Weaver cleared the bases with a grand-slam blast to left-center for an 11-1 lead.
"It was a fastball, middle-in," said Weaver. "I was dead set on that swing even though he had thrown eight straight balls before that. It was a good pitch to hit and I got barrel on the ball."
"Matt is a very good player," said Frady. "He has a lot of pop in his bat. If you take any of our players for granted, it's dangerous. The coaches and I talked today that if [Weaver] gets his right pitch, we could see something good. We didn't think of a grand slam, though."
Germany did not wait long to end the game by early termination, with five more runs in the sixth inning -- despite Frady bringing in his bench players. Maximilian Boldt, Mike Larson and Jake Shaffer all drove in runs with singles, and Simon Guhring got an RBI on a bases-loaded walk before Jendrick Speer was hit by a pitch to end the game.
David Hein is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.