Giants 6, Cardinals 5: Michael Morse homered and slugged a tiebreaking two-run double in the eighth inning as San Francisco knocked off host St. Louis in the opener of a four-game series.

Gregor Blanco went 3-for-4 and Pablo Sandoval homered and scored twice for the Giants, who won for the seventh time in eight games. Morse hit his decisive two-bagger off Trevor Rosenthal into the gap in right-center field to hand Carlos Martinez (0-3) the loss.

Javier Lopez (1-0) recorded two outs in the seventh and Jean Machi worked a perfect eighth for San Francisco. Sergio Romo allowed a run-scoring single to Matt Carpenter in the ninth before retiring pinch hitter Shane Robinson on a pop-up with runners on first and third for his 17th save in 19 opportunities.

Allen Craig hit a ground-rule double in the first for the Cardinals but San Francisco inched ahead in the second on Morse’s solo homer to left-center field and Blanco’s RBI single off Jaime Garcia. St. Louis scored twice in the fourth with Jon Jay’s run-scoring single to right-center field accounting for a 3-2 lead.

Sandoval hit a tying two-out homer over the fence in left-center field in the sixth before Craig put the Cardinals back ahead in the bottom of the inning with a blast to center off Ryan Vogelsong. Angel Pagan singled in the eighth and Blanco was originally held at third, but Martinez didn’t back up home plate and got in the way of catcher Yadier Molina’s vision as the throw by center fielder Peter Bourjos got away and Blanco trotted home to knot the contest.

GAME NOTEBOOK: Garcia struck out seven in seven innings while giving up three runs and five hits, and Vogelsong gave up four runs and seven hits in 6 1/3 innings. … Giants C Buster Posey (lower back) was held out of the lineup after a nerve condition again flared up. … Cardinals 1B Matt Adams (calf) missed his second straight start.