Japan thrashed Greece 6-1 to finish top of baseball's preliminary table and keep their gold medal hopes alive.
Erik Daniel Papas hit a home-run in the bottom of the seventh inning to salvage some pride for the hosts.
But the damage had already been done, Japan notching two two-run homers in the top of the seventh to clinch a semi-final place against Australia.
Cuba booked their semi-final place with a 5-0 win over Italy and will now play Canada, who humiliated Australia 11-0.
In Sunday's other game, Taiwan beat the Netherlands 5-1 to finish fifth in the group with three wins and four losses.