Photo courtesy of Philadelphia Union's Twitter account.
Andre Blake and Kacper Przybylko were the stars as the Union put one foot into the CONCACAF Champions League semi-finals.
A second-half brace from Przybylko and a late third from Anthony Fontana gave Philadelphia an unlikely 3-0 win away at the Mercedes-Benz Stadium.
Przybylko's first came from a Kai Wagner corner as his cross towards the front post was flicked into the path of the striker, who slotted home at the back post. The forward grabbed his second in the 72nd minute, thanks to the hard work of Leon Flach. The midfielder regained possession and led the break towards the goal before laying it across to Przybylko, who slotted it past Brad Guzan.
Fontana's goal came from the unselfish Przybylko, who allowed the midfielder an open goal to pass into to make it three as opposed to going for the hat-trick.
Jim Curtin made one change from the side that lost to Inter Miami at the weekend, as Cory Burke made his first start of the season and Sergio Santos dropped to the bench.
Atlanta United star forward Josef Martinez led the line for the hosts as they hoped for a first-leg lead.
Andre Blake did brilliantly denying Jurgen Damm in the sixth minute. The right-back got in behind the Union defence and had the goal at his mercy. He was met by the onrushing Blake, who quickly closed him down and did well to get a strong foot on the shot.
The visitors struggled for possession during the early stages of the game as Atlanta had them pinned inside their own half. However, Philadelphia stood strong against the storm, only giving the hosts half chances.
In the 32nd minute, Emerson Hyndman came close. A United corner went out as far as the midfielder who struck it on the half-volley first time. It took an acrobatic save by Blake to push it over the crossbar to deny the midfielder.
Blake was the Union hero again just minutes later. Marcelino Moreno was played through on goal by Martinez, and the winger was one-on-one. Blake was once again brave and made himself big, and he made solid contact to stop the ball from going past him.
Blake denied Martinez moments later as his left-footed shot towards goal was parried away as far as an Atlanta winger, who could only smash the rebound into the side netting.
Olivier Mbaizo almost did brilliantly to create a big chance for the Union on the stroke of half-time. The ball out to the right-back allowed him to run towards the penalty area, and he eventually got the better of the defender. Unfortunately, Mbaizo's cut back had no one on the end of it.
HALF-TIME: Atlanta United 0-0 Philadelphia Union
Moreno came close to scoring during the early stages of the second half. Atlanta hit the Union on the break, and Damm led the charge. He cut the ball back onto his right foot and laid the ball off to Moreno, whose effort was met by a solid hand from Blake.
Kacper Przybylko had a huge chance to grab Philadelphia the lead. Jamiro Monteiro played a smart ball to put the striker through, but his stab towards goal was closed down by Brad Guzan. The rebound hit an Atlanta defender and went out for a corner.
From the resulting corner, Przybylko made up for the guilt-edged miss as he gave the Union a shock lead. Kai Wagner's corner towards the front post was flicked on by Jakob Glesnes, and Przybylko snuck round to the back post to slot the ball into the back of the net.
Santiago Sosa almost equalised just short of the hour. The ball played back to the midfielder on the edge of the area allowed him to curl a first time effort towards the goal. The shot rattled the crossbar as the Union had a fortunate escape.
Przybylko had his second in the 72nd minute. There was a mix up as Atlanta tried to play it forward, and the Union regained possession. Leon Flach was able to drive towards the goal before laying it off to an unmarked Przybylko, who slotted it beyond Guzan and into the bottom corner.
Philadelphia had a third in the 86th minute. Half-time substitute Sergio Santos played Przybylko through on goal, and he ran towards the edge of the box. Another substitute in Anthony Fontana was alongside the striker as he continued to drive. Despite being on a hat-trick, the forward laid it off to Fontana, who passed it into an empty net.
In stoppage time, Santos forced Guzan into a save. The forward was played down the line, and he dribbled towards the goal before shaping to shoot. His left-footed effort towards the far corner was easily saved by Guzan.
FULL-TIME: Atlanta United 0-3 Philadelphia Union
Atlanta United: Guzan, Damm, Robinson, Franco, Bello, Sosa, Ibarra (Torres 68'), Hyndman, Barco, Martinez, Moreno (Mulraney 58')
Unused Substitutes: Lundgaard, Walkes, Lopez, Mulraney, Gurr, Torres, Ambrose
Philadelphia Union: Blake, Mbaizo, Glesnes, Elliott, Wagner, Martinez, Bedoya, Flach (Fontana 78'), Monteiro, Burke (Santos 46') , Przybylko (Real 87')
Unused Substitutes: Freese, Findlay, Sullivan, Collin