Frankie Hejduk and Landon Donovan scored late in the first half but rather than "managing the game" as Coach Bob Bradley refers to the process of protecting a lead, the USA conceded a superb goal to Iain Hume, and only an atrocious offside decision in stoppage time denied the spirited Canadians an equalizer and 30 more minutes.
The USA, 2-1 winners, also lost two players for Sunday's final against Mexico to suspensions. Hejduk incurred his second caution of the knockout phase, and Michael Bradley received a straight red card for a dangerous slide tackle from behind.
Donovan prompted several promising attacks, laid the ball back that Hejduk struck first-time for his first international goal in seven years, and put away a penalty earned by DaMarcus Beasley for the second goal. But when the Americans needed someone to regulate the tempo, to hold the ball and force Canada to foul and chase, they were, once again, found wanting.
Donovan again lined up on the right side of midfield, with Beasley on the left and Michael Bradley and Pablo Mastroeni paired in the middle. Left to right across the back were Jonathan Bornstein, Carlos Bocanegra, Oguchi Onyewu, and Hejduk. Up front were paired Clint Dempsey and Eddie Johnson.
Kasey Keller got the start in goal for his 100th cap, and Bocanegra, who played four seasons for the Chicago Fire before departing for Europe, served as captain.
The use of Dempsey as a forward in this tournament has borne fruit. He played a good ball into space that released Beasley to earn the penalty kick and he also nailed a header off a corner kick that Iain Hume miraculously cleared off the goal line with a back flick of his foot. His swagger and daring unhinges defenses and frees Bradley to use other options, such as Donovan, in midfield.
But Johnson again played tentatively. He refused to take defenders on in the attacking third when isolated one-on-one, and on his only shooting chance dribbled the ball to the keeper. He trapped and collected the ball fairly well, but when opportunities to knife into the box or combine with teammates arose, he wavered.
Beasley broke through early in the second half but rather than taking the chance himself, played a ball that Donovan whiffed attempting to slot home from a tight angle.
Midway through the second half, after Hume had come into the match, Hejduk and Dwayne De Rosario came up squabbling after colliding going for a high ball, and both were cautioned. The momentum had shifted and soon De Rosario, Hume, Julian De Guzman and Atiba Hutchinson were dictating the tempo.
Pablo Mastroeni played a strong game in the middle but faltered in the last 20 minutes, when the USA needed some stability and stamina.
Bradley played a few good passes and contributed some strong tackles, but his game was plagued by horrendous giveaways, and he erred badly by plunging into De Guzman in the 89th minute.
Bocanegra and Onyewu, like many teammates, were solid until the final minutes, when the USA struggled to clear the ball or keep possession when it turned over. Canada scored by countering after clearing a U.S. cross; Hume ran up the right flank and cut inside Bocanegra to curl a shot around Kasey Keller and just inside the post.
Benny Feilhaber replaced Johnson in the 80th minute and after shanking a shot badly, slid in at the back post to clear away a cross that rolled all the way across the goalmouth.
A misdirected header by Onyewu went right to Hutchinson in stoppage time, and he stuck it into the net, only for the goal to be wrongly disallowed.
The referee's assistant had raised his flag, incorrectly, on a shot from distance with the two most advanced Canadian attackers, one of which was Hutchinson, clearly not offside. They were in offside positions when Onyewu headed the ball but offside is judged when a member of the attacking team, not an opponent, plays the ball.
Bad offside decisions plagued the U.S. in several of its Gold Cup games. It got one back at its moment of greatest need. It can't count on such good fortune Sunday when it plays old rival Mexico in the final, but it has shown a tendency to win no matter what.
Gold Cup, Semifinal in Chicago, June 21
USA 2 Canada 1
Goals: Hejduk (Donovan) 39, Donovan (pen) 45; Hume (Hutchinson) 76.
USA -Keller, Hejduk, Onyewu, Bocanegra, Bornstein, Bradley, Mastroeni (Clark, 89), Donovan, Beasley, Johnson (Feilhaber, 80), 8 Dempsey (Twellman, 88).
CANADA -Onstad, Jazic, Hainault, De Guzman, Stalteri, Gerba (18- Nakajima-Farran, 81), Hastings, Hutchinson, De Rosario, Bernier, Nash (Hume, 64).
Corner Kicks: 3-4
Yellow cards:USA - Bornstein 20, Bocanegra 29, Donovan 41, Hejduk 60. CAN - De Rosario, Onstad 44.
Red card: USA - Bradley 89.
Referee:Benito Archundia (MEX), 1st Asst.: Luis Camargo (MEX), 2nd Asst.: Ricardo Louisville (SUR), Fourth Official: Walter Quesada (CRC)