ATLANTA - Julian Gressel scored early, Josef Martinez added a late goal and defending champion Atlanta United played keep away in between to beat the Union 2-0 on Thursday night to advance to the MLS Eastern Conference final.
United will host Toronto FC on Wednesday night. Toronto beat top-seed New York City FC 2-1 on the road Wednesday night.
Atlanta was especially diligent and crisp passing the ball, connecting on 85.4 percent of their 254 passes, and held the ball more than 53 percent of the time.
Left midfielder Pity Martinez, who returned to the starting lineup after missing United's first-round win over New England, came onto a ball about 35 yards in front of goal and sent a diagonal pass right ahead of Gressel.
The ball entered the box a few yards before Gressel, but he got to it before Union goalkeeper Andre Blake and touched it left around him to the far post for the score.
"We were secure with our passing, but also our opponent gets more tired and they have to run more behind the ball," United coach Frank de Boer said.
The Union, who were missing leading scorer Kacper Przybylko because of a foot stress reaction, pressed forward when they could, but United held the ball often and Atlanta also won 58 of 99 duels.
Just three of the Union's 11 shots were on goal, and the Union's best chance to score may have come in the 58th minute, when forward Jack Elliott's near header off of a corner kick sailed over goal.
Atlanta keeper Brad Guzan also repelled a quality shot by the Union's Marco Fabian in the 75th minute.
Without injured defenders Miles Robinson and Michael Parkhurst, de Boer deployed four across the back line, including rarely-utilized Mikey Ambrose and Florentin Pogba.
They were rarely threatened.
"Also, what we showed today in playing four in the back is that we can adjust very quickly to another system, and that's difficult for an opponent," de Boer said. "It was a very mature game."
Gressel assisted on the Josef Martinez goal in the 80th minute with a long ball from Atlanta's end as the striker got forward left for a breakaway. He the curled a left footer into the top left corner of the goal, outside of Blake's reach, for his seventh goal in seven career games against the Union.
Blake kept the match close when he made consecutive saves on close-range headers by Franco Escobar and Gressel in the 61st minute.
The Union appeared fatigued down the stretch, however likely because they last played Sunday in a 120-minute overtime win over New York Red Bulls, while United played a regulation match Saturday. Union coach Jim Curtin used all three of his subs.
Next Wednesday's Eastern final will match the defending MLS champion Atlanta and 2018 titlist Toronto. The teams split meetings in the regular season, United winning 2-0 at home on May 8 and FC prevailing 3-2 in Canada on June 26.