A complete and total meltdown.

That’s how the 2021 Giants will look back on Opening Night, when starter Kevin Gausman cruised through 6 2/3 innings of one-run ball and the lineup hammered three early home runs en route to a five-run lead.

The Giants, of course, couldn’t finish the job.

In a disastrous six-run eighth inning in which three Giants relievers combined to give up three hits, three walks and a hit by pitch, Gabe Kapler’s club lost control en route to a disastrous loss that ruined what should have been a banner night.

Pinch-hitter Alex Dickerson bailed out the Giants with a solo home run on an 0-2 pitch to kick off the top of the ninth, but the inevitable feeling of misery was only delayed.

In the bottom of the 10th, new Giants left-hander José Álvarez walked all three hitters he faced, allowing Evan White to score the game-winning run in a crushing 8-7 extra-inning defeat.

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In a dominant outing that may have been his best start with the Giants, Gausman recorded 20 outs, allowed two hits and struck out six Mariners hitters by relentlessly attacking the strike zone with his fastball. His performance was buoyed by Evan Longoria, Buster Posey and Austin Slater, who all drilled early home runs off Mariners starter Marco Gonzales, giving the pitching staff a five-run cushion to play with against a Seattle club that was missing its best hitter, Kyle Lewis, due to injury.

The Giants were in control, yet in stunning fashion, the game slipped away.

Each of the Giants’ first three relievers, Caleb Baragar, Matt Wisler and Jarlín García, walked the first batter they faced. Right-hander Tyler Rogers, who was called on to clean up an eighth-inning mess, gave up a double to Seattle second baseman Dylan Moore on the first pitch he threw.

After hitting the next batter, Jake Fraley, in the shoulder, Rogers induced the type of groundball that could have kept the Giants’ hopes of securing an Opening Night win alive.

But even the Giants’ infielders couldn’t throw strikes.

First baseman Brandon Belt handled a potential 3-6-1 double play ball and tossed a slider to shortstop Brandon Crawford covering second base. The errant throw trickled off Crawford’s glove and rolled into the outfield, allowing two Mariners to score and Seattle to take a 7-6 lead.

Dickerson’s home run injected life back into a dugout that was shellshocked by the Mariners’ rally, but the hope the Giants felt was short-lived.

The Giants’ eighth inning breakdown began when Wisler, a key free agent acquisition, entered with a five-run lead and allowed a walk, two singles and a run. García, who gave up one earned run in 18 1/3 innings last year, walked two of the three hitters he faced, including rookie Taylor Trammell, who came to the plate with the bases loaded.

With the game hanging in the balance, Kapler turned to Rogers, who he mentioned as a top candidate to begin the season as the Giants’ closer. A first-pitch double, a hit batsman and the catastrophic throwing error from Belt gave Seattle the lead, and didn’t exactly inspire confidence in the Giants’ ability to shut the door moving forward.

The disaster at T-Mobile Park will only lead to more questions about Kapler’s bullpen management, which has come under fire in his three-plus seasons as a major league manager. Yet considering the circumstances and the track records of each of the pitchers he called on in the eighth inning, it’s unlikely Kapler would have made different decisions if given a second chance.

Wisler allowed three runs last season for the Twins and was charged with three earned runs on Thursday. García gave up just one earned run with the Giants last year and surrendered two against Seattle. Rogers, who was called on to induce a groundball, finally did his job against the third batter he faced, but the right-hander’s defense failed him.

The blame for Thursday’s loss is widespread, but pointing fingers isn’t the Giants’ primary concern. After watching their bullpen blow up in Seattle, Kapler and Co. must find a solution and find one fast.

One down, 161 to go. Surely, the Giants can’t look worse than they did during Thursday’s collapse.

They hope they won’t find out.