Giants 5, Athletics 4: Brandon Belt had a two-run double and Hector Sanchez added the go-ahead single as San Francisco used a four-run ninth inning to upend Oakland in the series opener.

After Athletics closer Ryan Cook (2-2) walked the first two batters in the frame, Belt tied the game with a shot to left field that eluded Collin Cowgill. Cook was replaced by Sean Doolittle, who gave up RBI singles to Sanchez and Gregor Blanco.

Cook, who had allowed just two earned runs in 31 2/3 innings, saw his ERA climb from 0.57 to 1.71 after failing to retire any of the four batters he faced.

Santiago Casilla served up a solo homer in the bottom of the ninth to Josh Reddick before recording his 20th save for the Giants, who snapped a six-game road losing streak against Oakland. Clay Hensley (3-3) notched the win.

The Athletics grabbed an early lead with three runs in the first inning off Tim Lincecum, who remained winless since April 28.

Coco Crisp began the game with an infield single and stole two bases before scoring on Jamile Weeks’ single. After Weeks came around to score on Seth Smith’s ground ball, Lincecum walked Brandon Inge with the bases loaded.

Lincecum struck out the next three batters to end the first. He only allowed two more baserunners before exiting after six innings.

The Athletics, who had just two hits after Reddick’s first-inning single, lost for only the second time in their last 10 games.

GAME NOTEBOOK: Giants CF Angel Pagan was scratched from the starting lineup with a strained abdominal muscle. … The Giants started Hector Sanchez at catcher and had Buster Posey serve as their designated hitter. … Reddick entered the game hitless in his last 19 at-bats. … The Giants improved to 2-29 when trailing after seven innings. … The Athletics’ bullpen had a streak of 27 1/3 consecutive scoreless innings against the Giants in Oakland dating back to 2008 before Belt’s two-run double in the ninth.