The Union weren't beaten as badly as other teams have been at Atlanta United this year, but their 3-0 defeat Wednesday night at Mercedes-Benz Stadium in Atlanta was plenty thorough.

"We got outplayed by a better team tonight," Union manager Jim Curtin said. "Atlanta was sharp on the night from the opening whistle."

Atlanta's first two goals came in quick succession during the middle portion of the first half.

Julian Gressel, a contender with Union defender Jack Elliott for MLS rookie of the year, got the first. In the 27th minute, he cut past Warren Creavalle and Oguchi Oneywu and slid a low shot to Union goalkeeper Andre Blake's far post.

Josef Martínez scored the second, capping off a lightning-quick build up through the middle of the field with a sharply angled finish in the 33rd minute.

"In the first half, I thought we gave them too much time and space, maybe a little too much respect because they have been blowing teams out here and really outscoring them," Curtin said. "They punished us, because they have quality on their team."

Hector Villalba appeared initially to score Atlanta's third in the 55th. But the goal was overturned by the video replay booth because Martinez, who set Villalba up, was offside when he got the ball.

For most of the second half, the Union (9-13-9, 36 points) played with more spirit than Atlanta (15-8-7, 52 points). It helped that Atlanta sat back for much of the period, rarely feeling like its lead was truly troubled. Still, the Union registered three shots on goal and five overall after halftime, which — it has to be said — is more than they've had in some games this year.

The Union came inches away from pulling a goal back in the 67th, when Haris Medunjanin curled a shot from 20 yards off the crossbar.

Elliot came closest to scoring overall, forcing a sharp save from Brad Guzan with a volley off a corner kick in the final seconds of the game.

"Guys were a little more confident on the ball — obviously, it's a little easier when you're down 2-0 to be brave and be confident," Curtin said of his team's improvement after halftime.

Atlanta's third goal came in the 88th minute, when Jacob Peterson dribbled around an onrushing Andre Blake and finished from close range.

Blake finished the night with seven saves, including a few spectacular ones. But he could only do so much against the onslaught in front of him.

Atlanta clinched a playoff berth with its win, becoming the first MLS expansion team since 2009 to reach the postseason in its debut campaign.