SAN FRANCISCO -- Jake Smolinski had it all figured out.

The Oakland Athletics reserve outfielder would pinch-hit against San Francisco Giants left-hander Javier Lopez late in Tuesday night's game and take him up the middle for a hit.

Turns out he was right ... except for one little thing.

Smolinski didn't go up the middle for a hit. He went deep to left field for a three-run, pinch-hit home run in a five-run eighth inning, rallying the A's to a 13-11 victory over Giants in the second game of an interleague series.

"When I saw Lopez up (in the bullpen), I figured I'd get a chance," Smolinski said of the eighth inning, which began with the A's trailing 8-5. "I watched game film on him. I knew what to look for. I had a pretty good idea what was going to happen.

"Fortunately, it worked out."

Marcus Semien and Jed Lowrie also drove in runs in the game-swinging eighth for the A's, who recorded a two-game sweep of the San Francisco portion of the four-game set between Bay Area rivals.

The two-site series moves to Oakland for games Wednesday and Thursday.

"Great game. So many big hits. It seemed like two games," A's manager Bob Melvin said. "It'll definitely be a nice bus ride home."

On the strength of three multiple-run innings, the Giants found themselves up three in the seesaw affair before Khris Davis, who earlier homered, and Stephen Vogt began the eighth with consecutive walks off the fifth San Francisco pitcher, right-hander Cory Gearrin.

Semien's RBI single made it 8-6 and ended Gearrin's night in favor of Lopez (0-2), who retired pinch hitter Josh Phegley on a sensational catch by center fielder Denard Span before serving up Smolinski's homer that put the A's on top 9-8.

The pinch-hit homer was the first of the 27-year-old's career. Smolinski was just 4-for-21 (.190) in previous pinch-hitting efforts.

"I was pretty sure I got it. I knew I connected," Smolinski said of his fourth homer of the season. "You have to have an idea when you might be coming in, and be ready because it happens fast."

Lopez hadn't allowed a hit to a right-handed hitter all season.

"Just a rough night for the 'pen," Giants manager Bruce Bochy said. "It's hard to give up 12 runs in the last four innings and win a ballgame. They're big boys. They have to deal with this like all of us do."

The A's weren't done in the inning. Coco Crisp doubled and Lowrie singled to cap the five-run uprising.

The Giants got the potential tying run into scoring position in the bottom of the eighth after Ramiro Pena singled in Brandon Crawford, who had a five-RBI night, to close the gap to 10-9.

But right-hander Ryan Madson induced Conor Gillaspie, who had been 4-for-4, to pop out to second base, retaining Oakland's one-run lead.

The A's made Madson's job easier by adding three runs in the ninth, two on a double by Phegley and the last on an RBI single by Crisp.

The tack-on runs proved critical when Jarrett Parker and Span connected for back-to-back solo homers to lead off the bottom of the ninth. The home runs were the fourth of the season for each.

Madson then walked Angel Pagan to bring the tying run to the plate with no outs, but he got Brandon Belt on a fly ball and Buster Posey and Crawford on strikeouts to end the 3-hour, 53-minute marathon and earn his 15th save.

"When you're fighting in games like that, everybody comes into play," Melvin said. "And they did tonight."

Right-hander Zach Neal (1-1), the fifth of six A's pitchers, worked a two-hit, no-run seventh inning to get the win.

"This was one of those wild games," Bochy said. "You have one of these games once in a while. Hopefully you come out on top, but we didn't."

Davis, Vogt and Semien all scored three runs for the A's, who have won five of their last six.

The win allowed Oakland (34-43) to increase its all-time lead against San Francisco in the series to 56-52.

Gillaspie had four hits and Crawford three for the Giants (49-30), who for the second night in a row failed to win a 50th game.

The loss was just San Francisco's fourth in their last 17 games and gave the Giants a two-game home losing streak for the first time since May 10. The Giants fell to 5-5 in interleague play this season.

Parker, Span, Pagan and Pena added two hits apiece for the Giants, who out-hit the A's 18-12.

NOTES: The A's have hit a home run in seven consecutive games. ... Giants 3B Conor Gillaspie's four hits tied a career high accomplished twice previously. ... The Giants lost for the first time since Aug. 25, 2010, when scoring 10 or more runs in a game. ... The Giants scratched 2B Joe Panik from Tuesday's original starting lineup after he complained of concussion symptoms before the game. Panik was hit by a pitch in the helmet June 18 in a game at Tampa Bay. ... In order to create a roster spot for RF Josh Reddick, who came off the disabled list, the Athletics optioned 3B/OF Max Muncy to Triple-A Nashville. ... Tuesday was the first day since May 9 that Oakland had fewer than 10 players on the disabled list. ... A's manager Bob Melvin announced before the game that LHP Sean Manaea (strained left pronator) would be reinstated from the DL to pitch Wednesday's game against the Giants, and LHP Rich Hill (strained left groin) will be activated from the DL on Saturday to pitch that day against the Pittsburgh Pirates.