Lionel Messi’s first MLS playoffs began with momentum and mammoth expectations. They ended Saturday with a stunning upset and a GOAT’s whimper.

Messi and Inter Miami, the history-making top seed, crashed out in the first round with a second consecutive loss to Atlanta United — who barely snuck into the playoffs with a losing record and negative goal differential.

Messi, who’d been silent in a Game 2 loss, got his first playoff goal to level this decisive Game 3 at 2-2. But Atlanta responded; Bartosz Slisz snatched the lead back with a 76th-minute header; and the visitors sent shockwaves through American soccer with a 3-2 upset.

Miami entered these playoffs as the clearest favorite in MLS history. They waltzed to a victory in Game 1 of a three-game series, and seemed certain to advance. All they had to do was win one of two against an Atlanta team that had triumphed in less than a third of its regular-season matches — only 10 of 34.

But then, in Game 2, the Herons went silent in Atlanta.

Their 2-1 loss set up a “do-or-die” Game 3, Messi’s first in MLS.

Or, at least, that’s how English speakers described it. In Spanish, there’s a similar but different phrase. “Es un partido de matar o morir,” Inter Miami midfielder Federico Redondo said this week. Not “do-or-die,” but rather “kill-or-be-killed.”

And, in a staggering Saturday twist, it was Atlanta who did the killing.

The No. 9 seed, who won a play-in game on penalties, weathered an early Miami storm at Chase Stadium. Then, immediately after an early Matías Rojas goal, Atlanta’s Jamal Thiaré punished some lax Inter defending. He scored twice in three minutes, and the underdogs carried a 2-1 lead into halftime.

In the second half, Messi rose to meet the moment — temporarily.

But Atlanta answered.

And at the other end of the field, goalkeeper Brad Guzan, at age 40, made some heroic saves. He clawed away a close-range Jordi Alba header. He frustrated Luis Suarez and Messi. He made seven stops in total. He was, without a doubt, the man of the match and of the series.

Messi and friends were constant threats, but not the inexorable force they’d been at times in the GOAT’s first full MLS season. Suarez dove in search of a penalty but got no call. Messi smashed two free kicks into Atlanta walls. His shoulders seemed to slump in frustration.

And at the final whistle, with a blank stare, he trudged down the Chase Stadium tunnel.

Guzan and defenders chest-bumped in celebration.

And Miami’s title hopes died.

Their loss opens up an already-gaping Eastern Conference, which lost the No. 2 seed Columbus Crew last weekend and the No. 3 seed FC Cincinnati earlier Saturday.

It’s also a massive loss for MLS, whose playoffs lose significant luster, and will come out of the international break without their overarching narrative.

It doesn’t invalidate Miami’s 74 points, the best regular season in league history. “We celebrated the achievement, because it was something difficult and important,” Messi said in a recent interview.

“But,” he acknowledged, “we are aware that the real title we want is the MLS [Cup].”

Now, he might have only one more year to chase it.