SAN DIEGO -- Catcher Buster Posey hit a tiebreaking two-run double as the San Francisco Giants ravaged the San Diego Padres' bullpen Wednesday, and Matt Cain picked up his second win in four starts since returning from the disabled list as the Giants beat the Padres 7-1 at Petco Park.

The Giants have won three of the right-handed Cain's starts since he returned to the rotation July 2 after missing the first half of the season with a strained flexor tendon.

"The biggest thing is getting back out there and having fun with it," said Cain, who held the Padres to one run on five hits and a walk while striking out five in six innings. He was in a 1-1 deadlock when the Giants scored three in the top of the seventh after he came out for a pinch hitter.

"Everything is coming back together," Cain said. "It's just getting back out there, getting back into a routine and giving these guys a chance to win."

"That was a great job by Matty," Giants manager Bruce Bochy said. "He mixed it up well. He had a good breaking ball, and his fastball moved around really well. He hit his spots."

But he wasn't the only pitcher hitting his spots.

Padres right-hander James Shields allowed three hits and three walks over five shutout innings, striking out seven. But it took him 96 pitches to get through the fifth; and, when his spot came up in the batting order in the bottom of the fifth, Padres interim manager Pat Murphy sent up a pinch hitter.

"It was a combination of things," Murphy said. "He worked hard for five innings, he was leading off in the bottom of the fifth and he was coming up against a certain part of their lineup. It was a baseball decision. Shields understood."

Shields said he understands the National League strategy. But he wasn't tired: "I was still good to go; there was plenty more in the tank."

The Giants tied the game the inning after Shields departed on first baseman Brandon Belt's sixth-inning solo homer against Padres right-handed reliever Marcos Mateo. San Francisco broke it open with three runs in the seventh and eighth against right-handed relievers Brandon Maurer and Dale Thayer.

The Giants won their second straight and eighth in the last nine games.

The teams were tied 1-1 after six innings on the exchange of solo homers by Padres second baseman Jedd Gyorko and the Giants' Belt.

Center fielder Angel Pagan opened the Giants' seventh-inning, game-winning rally with a one-out single, although the key play ahead of Posey's double came on a full-count pitch to third baseman Matt Duffy.

Duffy, who had struck out in his three previous at-bats, checked his swing on a low outside slider from Maurer to draw a walk to extend the inning and place Giants at first and second. Posey then hit a sinking liner between Padres right fielder Matt Kemp and the right-field line that rolled to the wall after Kemp's unsuccessful attempt at a diving catch. Pagan scored easily and Duffy scored from first as Kemp retrieved the ball.

Posey then scored when right fielder Hunter Pence's line drive to right center landed between Kemp and center fielder Melvin Upton Jr. for a single.

The Giants added three more runs in the eighth against Thayer.

Shortstop Brandon Crawford opened the inning with his second homer of the series and his 14th of the season -- the Giants' second in as many nights against Thayer. Left fielder Gregor Blanco followed with a double down the line in right and scored on pinch hitter Justin Maxwell's single to center through a drawn-in infield. Duffy singled home the final run.

Gyorko put the Padres on the board first with a 378-foot line drive that hit off the lower scoreboard on the second deck in left. Gyorko hit a full-count pitch from Cain.

The Giants put runners on base in each of the first four innings against Shields and left the bases loaded in the fourth. Four of the seven Giants stranded by Shields were in scoring position -- three moving into position with one out.

Mateo retired the first two Giants he faced in the sixth before Belt connected on a 407-foot shot into the sand beyond the fence in right-center. It was Belt's 10th homer of the season.

NOTES: The Giants activated LHP Jeremy Affeldt from the disabled list and optioned LHP Josh Osich to Triple-A Sacramento. Affeldt, who had been on the disabled list since June 11 with a shoulder strain, faced two hitters Wednesday and allowed a hit and a walk. ... The Padres recalled OF Abraham Almonte from Triple-A El Paso and optioned INF Will Middlebrooks to the Chihuahuas. Although he had been used recently at shortstop, Middlebrooks will primarily play third base with El Paso. "We'd like him to come back as our third baseman," said manager Pat Murphy, who said Middlebrooks was going out to get his bat (.212) going. ... Padres RHP Brandon Morrow, who has been out since May 3 with right shoulder inflammation, threw a bullpen session Wednesday and will make a three-inning, 45-pitch rehab start with El Paso this weekend in Salt Lake City. ... Padres LF Justin Upton missed a third straight start with soreness in his left oblique muscle.