Before the Giants broke camp and headed to Seattle for Thursday night’s season opener, manager Gabe Kapler addressed his squad and outlined his vision for the club’s three core pitching philosophies: pound the zone, push the pace and know your plan.

In their first game of 2021, the Giants ended up beating themselves by struggling to stick to the first tenet.

A brilliant start by Kevin Gausman and a triumphant return to the lineup by Buster Posey were spoiled after the Giants’ bullpen blew a five-run lead in a disastrous eighth inning, culminating in a stinging 8-7 walk-off loss to the Mariners in 10 innings on Opening Night at T-Mobile Park.

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Giants relievers fanned the flames by issuing six walks over the final three innings, including three consecutive free passes by left-hander José Álvarez to force in the game-winning run in the bottom of the 10th.

“Not being able to throw strikes, that’s going to hurt you at the Major League level,” Kapler said. “We’ve got to do a better job. That was definitely the story of tonight’s baseball game.”

The Giants jumped out to a 6-1 lead following a trio of home runs from Posey, Evan Longoria and Austin Slater, but the tide quickly turned after Gausman departed following 6 2/3 innings of one-run ball.

Gausman struck out six and held the Mariners to two hits in the longest start of his Giants career, but Seattle subsequently erupted for six runs against relievers Matt Wisler, Jarlín García and Tyler Rogers in the eighth. The Mariners sent 11 batters to the plate in the inning, pulling within one on a two-run double by Dylan Moore before going ahead with the help of a throwing error by first baseman Brandon Belt.

With one out and runners on the corners, Belt fielded a ground ball off the bat of José Marmolejos and tried to throw to second to initiate a potential inning-ending double play, but the ball sailed wide of shortstop Brandon Crawford, allowing two runs to score to give the Mariners a 7-6 lead.

“There was no doubt, it definitely would have been a double play,” Belt said. “I'm not really sure what happened. I feel like I did everything right and just kind of yanked it a little bit. It's a play I feel like I’ve made 99 times out of 100. It's unfortunate that it happened at the time it did.”

Alex Dickerson briefly bailed out the Giants by launching a game-tying, pinch-hit home run off Mariners closer Rafael Montero in the ninth to force extra innings. Still, the reprieve proved fleeting. Longoria opened the top of the 10th at second as the automatic runner, but the Giants were unable to score after Anthony Misiewicz retired Belt, Wilmer Flores and Posey in order to end the inning.

Álvarez took over in the bottom of the 10th, but he was unable to retire any of the three batters he faced and issued a bases-loaded walk to Jake Fraley to end the game.

Veteran Jake McGee was the only Giants reliever to post a 1-2-3 inning on Thursday and needed only eight pitches to get through the bottom of the ninth, but Kapler said he views the 34-year-old left-hander as a “sprinter” for now and didn’t consider sending him back out for another inning of work despite his low pitch count.

“Games like this are always frustrating,” Belt said. “We had a great start to the game, obviously, and we kept tacking on, too. It’s not like we just gave up or something. Everybody's out there trying to do the best they can. We just came up short tonight. Obviously, it's not fun, but we're going to do everything we can to make sure it doesn’t happen again.”

The late-inning meltdown overshadowed several positive developments from earlier in the night, most notably the performance of Posey, who returned to the Giants’ lineup for the first time in over 18 months after sitting out the 2020 campaign due to COVID-19 concerns.

In his first at-bat of the season in the second inning, Posey announced his return by driving a 2-0 cutter from left-hander Marco Gonzales out to left field to collect his first home run since Sept. 24, 2019.

Posey, 34, recorded a career-low .688 OPS in 2019, but the Giants are optimistic that he could be poised for a rebound now that he’s further removed from the right hip injury that sapped his power and led to season-ending surgery in August 2018.

“It was awesome just to see that again,” Belt said. “Two years ago, he wouldn't have been able to do that. It looks like his hip is a lot healthier now, and he's able to turn those pitches. You’re probably going to see a lot more of that from Buster this year.”