"It's completely baffling, that's all I can say," an upset Hackworth said after the game. "I don't know how a player or a coach can stay and play their game, manage what they have to manage, when that kind of stuff happens."
The Union got to the semifinal by winning their first Open Cup match in late May, defeating the Rochester Rhinos at home, 3-0, then going on to beat D.C. United, 2-1, and the Harrisburg City Islanders, 5-2, in the next two rounds.
Sporting Kansas City (11-5-3), the first-place team in MLS's Eastern Conference, had lost to the ninth-place Union, 4-0, at PPL Park on June 23.
"I think beating them 4-0 last game didn't really do us any good," forward Jack McInerney said. "We knew they were going to come here and play physical and look for fouls. There were a couple of scuffles here and there, but we needed to keep our composure."
A sloppy first half saw a lot of bad touches and physical play without many scoring opportunities. Each team had eight fouls apiece in the first 45 minutes of play.
"They disrupted us a little bit," Freddy Adu said. "When they got physical, when they hit us, and when they get away with some other stuff, it makes it a little bit difficult."
For the second straight game McInerney provided the best offensive chances for the Union. Michael Farfan's free kick into the box for a McInerney's header in the 31st minute was the Union's most dangerous scoring opportunity in the first half. The header was on target but just high, up and over the crossbar. In the 38th minute, McInerney sent a low blast wide of the net.
In the first half, Sporting Kansas City had two corner kicks and several free kicks just outside the 18, including five total shots. At the half, the game remained scoreless.
Amobi Okugo had a solid defensive game again for the Union, constantly clearing the ball and breaking up passes.
Antoine Hoppenot, who has been a spark plug off the bench for the Union in recent games, entered for Lionard Pajoy in the 64th minute. While he had several close chances, Hoppenot could not put one away.
In the 65th minute, Graham Zusi sent a free kick into the box and Jacob Peterson got a head on it, sending the ball into the top right corner of the net. The ball deflected off goalkeeper Zac MacMath's hands and into the back of the net.
"It just snuck in there, that's all," MacMath said of the goal.
"I know I can save that ball and I should save that ball."
Adu had a chance for an equalizer in the 89th minute after controlling the ball in the box, but he sent his shot sailing over the net.
Zusi put the game away for Sporting in stoppage time when a long blast from midfield went past MacMath, who had been playing up, about 40 yards off his line.
Kansas City advances to final against the winner of Wednesday's Seattle Sounders-Chivas USA game, which ended too late for this edition.