OAKLAND, Calif. -- The San Francisco Giants lost their previous four games, blowing leads in three of them, before facing the Oakland A's on Tuesday night.

This time, the Giants grabbed a five-run lead in the first inning and never looked back.

Nick Hundley, Brandon Belt and Hunter Pence hit home runs, powering the Giants to a 10-4 victory over the A's in the second game of the Bay Bridge Series at Oakland Coliseum.

"We've had the lead in three of these losses in the last four we had, and we couldn't hold on to them," Giants manager Bruce Bochy said. "That can beat you up a little bit, but tonight they looked like they were going to make sure we didn't let this one get away."

Hundley hit a two-run homer in the first inning, Belt a solo shot in the fourth and Pence a three-run blast in the seventh after the A's intentionally walked Buster Posey. Pence's shot made it 10-4.

San Francisco right-hander Jeff Samardzija, a former Athletics pitcher, allowed four runs (three earned) and six hits in a season-high-matching eight innings.

He won back-to-back games for the second time this year. Samardzija (6-11) struck out five, walked two and improved to 6-2 in 11 career starts at the Coliseum. He threw 118 pitches.

"Any time those guys are out there swinging like that, you just sit back and enjoy it and watch it, especially in these American League games when you're not hitting," Samardzija said. "And then you want to go out there and have quick innings and get those guys back in the dugout, because if they're swinging it like that, you want to keep those bats in their hands."

The Giants matched their season high for home runs and pounded out 14 hits.

"It was nice," Bochy said. "I can't tell you the last time we scored five in the first. It's nice to have that cushion. They came right back, and I thought what was important was we kept adding on. Of course, the three-run homer by Pence, that helped put it away."

A's left-hander Sean Manaea (8-6) gave up six runs (two earned) and nine hits in three innings. He struck out one and walked one.

Manaea, who threw 40 pitches in a five-run first inning, had pitched into the sixth inning or later in 12 consecutive starts before Tuesday.

"It was really disappointing," Manaea said. "I just wanted to keep things going. Unfortunately, tonight wasn't my night. They definitely smacked me around a little bit."

Belt went 2-for-5 with a double and three runs. He hit his 17th home run of the season, one off his career high, set in 2015.

Posey went 3-for-4 with a double, a run and two RBIs. Kelby Tomlinson had two hits, including a triple, and scored twice. Hundley had a double to go with his homer.

Matt Joyce hit a two-run homer in the fifth inning and drove in three runs for the A's.

The Giants scored five runs (one earned) against Manaea in the top of the first inning as they sent 10 batters to the plate.

Gorkys Hernandez lined a leadoff double down the left field line and scored on Tomlinson's single to right. Belt hit a ground ball to typically sure-handed first baseman Yonder Alonso, who booted the ball for an error as Tomlinson moved to second.

Posey followed with a hit-and-run RBI single to right, making it 2-0. With one out, Brandon Crawford brought Belt home with a ground ball to Alonso. Alonso got the force at second, but Crawford beat shortstop Marcus Semien's throw to first.

Hundley made it 5-0 with a two-run shot to left-center field, his fifth home run of the season.

"It's just one of those nights where he was off a little bit," A's manager Bob Melvin said of Manaea. "We gave them some extra outs in the first, but usually he's getting a few more swings and misses, and he really wasn't getting many."

Oakland scored a run in the bottom of the first when Semien singled with one out, moved to second on a walk to Khris Davis and scored on Alonso's single.

The Giants made it 6-1 in the second as Belt hit a leadoff double and came home on Posey's double, but the A's answered with an unearned run in the bottom of the inning.

Belt launched a leadoff home run to right in the fourth off reliever Michael Brady, increasing the Giants' lead to 7-2.

Oakland cut the deficit to 7-4 in the fifth when Jaycob Brugman hit a leadoff single and Joyce crushed a two-run shot into the right field seats for his 14th homer of the season.

NOTES: Giants RHP Johnny Cueto had an MRI of his right forearm that revealed a mild flexor strain, Giants trainer Dave Groeschner said. Cueto, who is on the disabled list because of blisters on his right hand, had to leave his rehab start Monday night for Class A San Jose after 34 pitches because of right forearm tightness. He will be re-evaluated after a week of rest and treatment. "This is a pretty good prognosis overall," Groeschner said. ... Oakland CF Dustin Fowler, one of three prospects the A's received from the New York Yankees in a trade Monday for RHP Sonny Gray, arrived at the Coliseum on Tuesday. Fowler is recovering from a ruptured right patellar tendon he sustained while making his major league debut June 29 for the Yankees. Fowler said he expects to resume baseball activities in December and be ready for the start of spring training next year. ... RHP John Axford, designated for assignment Thursday by the A's, cleared waivers and was released Tuesday.