SAN FRANCISCO -- Oakland Athletics rookie pitcher Chris Bassitt boldly predicted Friday that he would hit a home run against San Francisco Giants ace Madison Bumgarner when they went head-to-head Saturday.

Turns out Bassitt was right about one thing: The matchup produced a homer. He just had the wrong guy hitting it.

Bumgarner limited the A's to one run in seven innings and helped his cause with his third home run of the season, winning a pitcher's -- and hitters' -- duel with Bassitt while delivering the Giants a 2-1 victory.

"I'm just trying to hit the ball hard and contribute any way I can up there," Bumgarner said of his hitting. "It's been going pretty well of late, but I wouldn't read too much into it."

In beating their cross-bay rivals for the second consecutive day and 15th time in their last 18 meetings in San Francisco, the Giants posted a fourth straight win and seventh in eight games since the All-Star break.

Third baseman Matt Duffy singled in second baseman Joe Panik with the tiebreaking run in the sixth inning and the Giants' pitchers took it from there, with right-handers Sergio Romo and Santiago Casilla throwing a shutout inning each in relief of Bumgarner (11-5).

"Clutch hitting there," Giants manager Bruce Bochy said of Duffy. "That's what it takes to win a close game. And the bullpen, too."

Casilla overcame a walk and a single by reserve outfielder Billy Burns in the ninth to post his 25th save, matching his career high.

The win was the Giants' fifth in a row at home and improved their interleague mark to 8-3.

"We've been firing on all cylinders," Bochy said. "Today was just a well played game by both sides."

Bumgarner staked himself to a 1-0 lead in the third, belting a full-count Bassitt pitch into the bleachers in left field. It was Bumgarner's ninth career home run and the first given up to a pitcher by Oakland since 1972.

Bassitt only took half the blame for the bomb afterward.

"The pitch before was a strike. He (home plate umpire Joe West) missed it," Bassitt said of a 2-2 pitch that could have produced a strikeout. "It was an awkward thing. I was like, 'Don't walk him, don't walk him, don't walk him,' and I grooved a fastball right down the middle.

"I have to go after him like a hitter. It was a horrible mind-set and he made me pay."

Bassitt batted twice against Bumgarner, striking out once and hitting a tapper back to the mound.

"I hope I never have to do that again," he said of batting, which he insisted he hadn't done in a game since 2007. "It was a learning experience for me. I've got to do better than that."

Bumgarner might have had Bassitt's number, but he never solved A's second baseman Ben Zobrist, surrendering a single, double and triple in three at-bats.

But all came with two outs and Bumgarner retired a World Series rival, Oakland first baseman Billy Butler, on a fly ball and two popups to strand Zobrist each time.

"We needed one big hit and never got it," A's manager Bob Melvin said. "He (Bumgarner) made the big pitch when he had to."

The lone A's run came in the fourth on a double by catcher Josh Phegley that scored right fielder Jake Smolinski, who opened the inning with a single.

In winning his third straight start, Bumgarner gave up five runs and two walks in seven innings. He struck out seven.

"Those guys, there's not just one way to get them out," he said of his pivotal matchups with Butler. "You've got to give them your absolute best stuff."

Bassitt (0-3) took the hard-luck loss, allowing two runs and five hits in six innings. Promoted earlier in the day to help fill a hole in the A's starting rotation created by the trade of left-hander Scott Kazmir on Thursday, Bassitt struck out four and did not walk a batter.

"He threw well. Can't say I was surprised," Bochy said of Bassitt. "It's a little difficult when you haven't see somebody. But I don't want to take anything away from him."

The 26-year-old has not allowed more than two runs in any of his four major league starts this season. He lost two other 2-1 decisions to account for his previous two defeats and has received just six runs of support in his four starts.

Panik had a double and a single for the Giants, who totaled six hits against three A's pitchers.

The loss was the third in a row for the A's, who fell to 5-4 in interleague play and 7-19 when opposed by a left-handed starting pitcher.

NOTES: The last pitcher to hit a home run against the A's was Baltimore Orioles LHP Dave McNally off LHP Ken Holtzman in 1972. ... Three of Oakland's six hits went for extra bases. ... In order to recall RHP Chris Bassitt for Saturday's start, the A's demoted RHP Angel Castro to Triple-A Nashville. Castro was with the parent club for only a day and pitched one inning (two hits, no runs, one strikeout) in Friday' loss to the Giants. ... A's CF Billy Burns (testicular contusion) was held out of the starting lineup but pinch hit in the seventh inning and stayed on to walk to in the ninth. ... A's LHP Sean Doolittle (strained left shoulder) threw 15 pitches from 105 feet in the outfield before Saturday's game and stamped himself ready to be promoted to a bullpen session next week. ... Giants RHP Tim Lincecum (bruised right forearm and degenerative hips) appears to be on the same recovery path as Doolittle. He also is tentatively scheduled for a bullpen session next week after throwing on flat ground Saturday. ... The starting time of the Giants' game on Sunday, Aug. 23, at Pittsburgh has been changed from 1:35 p.m. ET to 8:08 p.m. ET in order for it to be televised nationally by ESPN.