Soccer’s true charm lies not in the grind of league competition, but in cup matchups such as Tuesday night’s tasty affair between a top-flight team (D.C. United) and a band of brothers with amateur status and big hearts (Baltimore’s Christos FC).

Full professionals squared off against a loan officer, a plumber, an x-ray technician and a sales rep in a pastoral little stadium shadowed by a barn and surrounded by endless rows of youth fields.

With hundreds of visiting supporters outfitted in neon-green T-shirts watching, Christos took a shock lead before running out of steam down the stretch and falling to the heavily favored MLS team, 4-1.

United did not go ahead until the 81st minute against its wilting foes. Lloyd Sam broke the tie, Julian Buescher scored his second of the night in the 90th, and Ian Harkes notched his first pro goal on the last touch of a compelling match before 5,286 at Maryland SoccerPlex in Montgomery County.

“As we grow as a soccer country,” D.C. Coach Ben Olsen said, “this is going to happen more and more — better quality amateurs out there that can make these types of runs. What’s great about the Open Cup is that these stories can happen.”

United advanced to the round of 16 in the 103-year-old tournament, which features American teams from all levels of soccer. Before the establishment of fully pro teams, the Open Cup was ruled by industrial and ethnic clubs, such as Bethlehem Steel, Robins Dry Dock and Brooklyn Italians.

Christos, a 20-year-old club sponsored by a discount liquor store located near BWI airport, was the last amateur team left in this year’s edition.

“These guys train every day,” said Christos co-coach Bryan Bugarin, whose team rarely practices. “You can just tell we had tired legs at the end. We knew we could play, soccer-wise. It was, ‘Hey, how long can we last, fitness-wise?’ ”

Mamadou Kansaye, one of several former UMBC players on the squad, scored in the 23rd minute. He acknowledged afterward, though, that maintaining United’s pace for 90 minutes was difficult.

“Fitness is lacking, big time,” he said, limping into a postgame news conference.

With a stretch of three MLS matches in eight days on the horizon, Olsen mixed regulars, such as Bobby Boswell, with reserves, most notably 17-year-old Chris Durkin.

United aimed to score early and watch the visitors energy wane in the thick evening air. Two early misses by Sebastien Le Toux, however, opened the door for Christos.

After forcing Travis Worra into a leaping save, Christos went ahead.

Kansaye, who coaches in Maryland’s Olympic development program, lifted a 19-yard free kick over the defensive wall. It bent away from Worra and settled into the right side of the net, triggering a mass of Christos supporters to charge down the grass knoll and celebrate in the corner with the players.

“It’s like, ‘Oh, here we go,’ ” Boswell said. “These games are tough. They’re riding momentum and, even though we’re the home team, it’s more of a home game for them.

“All week, it’s been David vs. Goliath. I had people sending me messages: ‘Have you seen this team? How cool is that?’ We’re playing them. It’s not cool to me.”

Remaining patient against a disciplined defensive bunker, United drew even in the 35th minute. Buescher, a 2016 first-round pick, broke the resistance with footwork and fired a low shot into the far corner.

Despite the departure of striker Pete Caringi III with a hamstring injury before intermission, Christos remained on even terms — on the scoreboard and in their performance.

As United searched for answers, Christos waited to counterattack. Worra was forced to make a diving save. United’s Lamar Neagle hit the left post from close range.

Four minutes after entering, Sam drove a low shot from distance into the far corner for a 2-1 lead. Exhausted and down a goal, Christos was done.

For good measure, Buescher and Harkes tacked on late goals.

“It’s a no-win situation,” Olsen said. “Everyone’s expecting us to win, and win big. I’ve got a lot of love for Baltimore. Their group should be very proud.”

United (4-7-3 in MLS) will resume league play Saturday at first-place Toronto FC.