Sporting KC will host the winner of the other semifinal game between Chivas USA and the Seattle Sounders on either August 7 or 8.
The victory by Sporting KC, tied for first in Major League Soccer's Eastern Conference with D.C. United, avenged their worst defeat this season, when they lost 4-0 to Philadelphia on June 23.
"Sometimes during the season, you have games like we had here," Sporting KC coach Peter Vermes said. "We had a chance to come back here and give a little payback to these guys. They handed it to us that night. I think the guys were locked into a result, and what they had to do to get a result. I don't think we were more physical than they were."
The game, however, was more physical than the last meeting. There were 38 combined fouls and six cards issued between the two teams.
"I don't think the physical aspect was a key to our game tonight," Zusi said. "The main key was defensively to remain more solid, lock down the passing lanes and make sure they didn't get behind as much as they did the first game. No team was any more physical than the other, and that's the way games go sometimes."
Two minutes after Peterson's goal, Philadelphia had a chance to tie the game, but the Union's Antoine Hoppenot was denied on a sliding save by Sporting KC goalie Jimmy Nielsen. In the 89th minute, the Union's Freddy Adu had another chance to equal the score but his shot from 12 yards out sailed high.
Peterson's goal came off a header and was set up on a free kick from Zusi. Union goalie Zac MacMath got his left hand on the shot, which deflected into the upper right corner of the net. Zusi scored in the 93rd minute.
Union coach John Hackworth was not happy with the physical play of Sporting KC.
"They did what they had to do to come in and get the result, different styles, different philosophies," Hackworth said. "They put in a good game plan and got some help executing it. It's completely baffling, that's all I can say. I don't know how a player or a coach can play their game and manage what they have to manage when that kind of stuff happens."
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