Giants 9, Rockies 2: Pablo Sandoval homered from both sides of the plate for the first time in his career and Buster Posey also went deep as San Francisco continued its domination of visiting Colorado by finishing a four-game sweep.

Barry Zito (13-8) struck out six and allowed two runs on 10 hits in 5 2/3 innings to win his fifth straight decision for the Giants, who reduced their magic number to clinch the National League West title to three. The Giants have won eight of nine and have a 10-game lead over the Los Angeles Dodgers, who fell 4-1 at Washington.

Sandoval and Posey hit back-to-back blasts in the six-run fourth as San Francisco broke open a tied game. The Giants have won six straight against Colorado and claimed the season series 14-4. San Francisco went 13-5 against the Rockies last season.

Colorado left-hander Jorge De La Rosa (0-1) made his season debut after being sidelined since May 24, 2011, after undergoing Tommy John surgery. He lasted just 3 2/3 innings, allowing six runs (five earned) on seven hits.

Hector Sanchez went 3-for-4 and scored a run and Marco Scutaro went 2-for-4 with two RBIs for the Giants.

GAME NOTEBOOK: Sandoval became the first Giants player to homer from both sides since Randy Winn in 2008. … Rockies rookie Wilin Rosario, making his first career start at first base, misplayed Sandoval's pop-up in the third, allowing a run to score. … Scutaro, who was acquired from Colorado on July 27, is 22-for-56 (.393) in 13 games against his former team since the trade.