The game was stopped after seven innings by mercy rule.
Germany will face Canada at 1 p.m. ET on Monday in the pool final, with the winner earning a berth in the 2013 World Baseball Classic.
Braves infield prospect Matt Weaver led the Germans, falling a triple shy of the cycle and going 3-for-4 with four RBIs. Phillies farmhand Aaron Altherr homered, singled and drove in three runs, while Reds prospect Donald Lutz -- a California League All-Star -- added a pair of singles.
Veteran left-hander Enorbel Marquez (1-0) went the distance for the winners, allowing an unearned run on a pair of hits. The Cuban-born 37-year-old, who pitches in Italy, struck out eight and did not walk a batter.
"The game was really important for us," Marquez told MLB.com. "I knew that we had to win. I just did my best and it worked out. The coach just told me we have to win to advance, and I did all I could."
Great Britain starter Kyle Wilson (0-1), a UCLA product who last pitched affiliated ball with Double-A Jacksonville in 2008, faced only three batters, all of whom scored.
His successors fared little better as Germany tacked on two runs in the second, four in the fifth, one in the sixth and six in the seventh.
Marquez cruised through the British lineup. Great Britain's first base runner came on a two-out error in the fourth and its first hit was Albert Cartwright's leadoff single in the seventh. The Phillies prospect scored Britain's lone run on a one-out single by B.J. Hubbert that was compounded by center fielder Max Kepler's fielding error.
Marquez faced three batters over the minimum.
"There were a couple of good hits, but our defenders were there," he said. "And then they just hit the ball and there was no one there. And that's it."
Germany fell to Canada, 16-7, on Saturday, in a game that last four hours and did not end until nearly midnight local time. The Germans hope to avenge that loss and punch a ticket to the World Baseball Classic on Monday.
"Last night, we were very disappointed [after losing to Canada]. But I knew we would come back," said German manager Greg Frady. "We took this game very seriously. The hitting was on point. Nothing we did surprised me today. It was a great victory for German baseball, and it's an honor to play against Canada for a spot in the World Baseball Classic."