The Rockies, who started the day five games back in both the National League West and the NL wild-card race, improved to 71-66.
Rockies starter Jeff Francis (15-6) went 5º innings, giving up three runs and nine hits. His 15 wins are the most by a Colorado pitcher since Shawn Estes had 15 in 2004.
Manny Corpas got three outs for his 13th save in 15 chances, and his 13th in a row since replacing Merced High grad Brian Fuentes as closer.
Colorado's big third inning came mostly at the expense of Giants starter Matt Cain. The Rockies roughed up Cain (7-14) for six runs and five hits in the inning before he was lifted in favor of reliever Pat Misch.
Francis started the rally with a one-out double and he scored on a single by Kaz Matsui. Matt Holliday's sacrifice fly scored Willy Taveras, and after consecutive walks to Todd Helton and Garrett Atkins loaded the bases, Brad Hawpe lined a two-run single to right field.
Misch came on and gave up an RBI single to Troy Tulowitzki and a triple to right to Iannetta.
Taveras had three hits and Helton and Iannetta had two hits each.
