Ethier's third inning shot was his 31st home run of the season. Belliard hit a two-run fourth-inning single.
Winner Randy Wolf (11-6) surrendered two runs and five hits in six innings.
Andres Torres hit two solo homers for the Giants, who face a 4 1/2-game deficit in their battle with the Rockies for an NL wild-card berth.
Losing pitcher Tim Lincecum (14-6) gave up five runs and four hits in four innings.