SAN FRANCISCO -- The Giants paid a steep cost for poor defense on Friday night.

A defensive miscue by first baseman Wilmer Flores opened the door for a stunning ninth-inning rally by the A’s, who erased a five-run deficit en route to a wild 8-7 win over the Giants in 10 innings in the Bay Bridge Series opener at Oracle Park.

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Stephen Piscotty crushed a game-tying grand slam off closer Trevor Gott in the ninth, and Mark Canha drove in the go-ahead run with a sacrifice fly in the 10th, sealing a painful loss for the Giants, who fell to 8-13 on the season following their ninth defeat in their last 12 games.

It marked the Giants’ first loss when leading by five or more runs in the ninth inning or later since June 25, 1929, against the Brooklyn Dodgers, according to Stats LLC.

“The processing happens tonight,” manager Gabe Kapler said. “The after-action review, the rewatching the game in your mind happens tonight. We talk about it and figure out ways to get better for tomorrow and things that we could have done differently and look at it from a ‘how can we raise the bar for ourselves’ perspective.”

The Giants carried a 7-2 lead into the ninth on the strength of home runs from Evan Longoria, Hunter Pence and Mike Yastrzemski, but Matt Olson ignited the A’s uprising with a one-out solo homer off Gott. After a walk by Mark Canha, Robbie Grossman hit a potential double-play ball to the right side that was fielded by Flores.

Flores took a step toward first base, but he never touched the bag and instead threw to second to try to get the forceout. Shortstop Brandon Crawford prematurely took his foot off second base, allowing Canha to beat Flores’ throw and preventing the Giants from recording an out on the play. Gott then hit Khris Davis with a pitch to load the bases for Stephen Piscotty, who crushed a misplaced breaking ball out to left field for a game-tying grand slam.

“The velocity was there and the command wasn’t,” Kapler said of Gott. “He came out with all the intensity that he usually does. He came out with all the aggressiveness that he usually does.”

Shaky defense has been a recurring theme for the Giants, who lead the Majors with 21 errors this year. Kapler used Crawford as a late-inning defensive replacement in the eighth, but the manager refrained from subbing in first baseman Brandon Belt, who is dealing with soreness in his left calf and right heel and was only available to pinch-hit.

Apart from wanting to rest Belt, Kapler said he trusted Flores to make plays at first base, which the Giants view as the 29-year-old infielder’s best position.

“It was an indecision. That was on me,” Flores said. “I put Crawford in that situation, he didn’t know if I stepped on the base or not. I thought I was closer to the base, but I was not.

“I should have just stepped on first base with that lead that we had.”

The Giants put a pair of runners on base with one out in the bottom of the ninth after Flores singled and Pence walked, but Crawford and Pablo Sandoval grounded out and the game headed to extras.

Matt Chapman opened the 10th at second base as the A’s automatic runner and advanced to third on a groundout before scoring the go-ahead run on a sacrifice fly by Canha. The Giants couldn’t respond in the bottom of the inning, as Liam Hendriks struck out Belt, Mauricio Dubón and Yastrzemski to end the game.

The Giants squandered a strong start from right-hander Johnny Cueto, who gave up two runs over seven innings in his longest outing of the season. He walked two and struck out five while throwing 104 pitches, his most since Sept. 19, 2017, against the Rockies.

“That’s baseball,” Cueto said. “You want things to go your way, but when they don’t, you just move forward.”