Playing in sweltering heat and humidity, United manager Alex Ferguson liked the way his team responded when they went down by a goal.
"It requires good patience, and we showed the patience," Ferguson said. "We don't panic being down, we just play our game."
A crowd of 61,308 saw the preseason debut for new United goalkeeper David de Gea, who allowed an early goal on a header by Chicago's Cory Gibbs in the 13th minute. But De Gea recovered to make some nice saves in his first action with his new team.
Ferguson said he initially thought De Gea was at fault on Chicago's goal, but changed his mind after seeing a replay.
"When I saw it again, he had absolutely no chance," Ferguson said.
United midfielder Ji-Sung Park said the heat was a challenge for players on both sides.
"When it's hot, it's always difficult," Park said. "But we played our style, and we won. It was a pretty good match."
Trailing at halftime after being turned away several times by Chicago goalkeeper Sean Johnson, United brought on some of its top players as substitutes to start the second half — including Rooney, one of the world's top players.
Rooney showed his ability to create an instant scoring chance when he chipped the ball over backup Fire goalkeeper Jon Conway, a second-half substitute, to tie the game in the 66th minute.
Then da Silva dribbled through the defense and used his left foot to flip the ball past Conway in the 75th minute. Nani sealed the victory with a goal in the 82nd minute.
While Chicago missed on a few chances to extend its lead, Fire interim coach Frank Klopas scoffed at the idea that his team let one get away.
"No," Klopas said. "Come on, guys, you've got to understand what we're playing against. We put up a very good challenge. For me, more than anything, we made the game competitive."
With temperatures in the 90s and oppressively high humidity in Chicago, Ferguson went with a young starting lineup — and it showed early on, as United's defense looked shaky at times and the Fire exploited it to take the lead in the first half.
Chicago took a 1-0 lead in the 13th minute when a long free kick by Marco Pappa was headed into the back of the net by Gibbs. Chicago went on to hold long stretches of possession for much of the first half and created a few more scoring chances — including a hard shot by forward Cristian Nazarit that went just wide of the goal in the 32nd minute.
It briefly appeared as though the Fire, who are near the bottom of the standings in the MLS' Eastern Conference, would save some face for the MLS after United trounced its first two opponents on its U.S. tour.
United beat the New England Revolution 4-1 and the Seattle Sounders 7-0 on Wednesday — although the Seattle game was close at halftime, with United leading 1-0.
"Chicago Fire played well," Ferguson said. "Very good in the counterattack, a lot of pace up front. They gave us some problems. They could have scored two or three more."
Saturday's game featured the much-anticipated debut of De Gea, who left Atletico Madrid to join the Premier League champions in the offseason.
He did not play in the first two legs of United's U.S. tour. He is expected to be the long-term replacement for Edwin Van der Sar, who retired at the end of last season, but hasn't been handed the starting job yet.
De Gea recently helped Spain win the European under-21 championship.
Ferguson called De Gea's technique "very good."
"When he needed to step up and make some key saves for his club, he did," Klopas said.
Klopas hoped his team's strong showing in the first half made a good impression on local fans who enjoy top-level European soccer but haven't been to a Fire match yet.
"I thought it was a positive impression," Klopas said. "We wanted to step on the field and play with confidence."
Ferguson, meanwhile, said he will begin getting his regular players ready for the season by allowing them to play 90 minutes in their remaining exhibition games, including Wednesday's MLS All-Star game.
And Ferguson said he doesn't expect United to add any more players in the summer transfer window.
"I think we'll be starting the season with the players I have," Ferguson said. "And I'm satisfied with that. There's nothing on the horizon at the moment, I can assure you."