ATLANTA » Jamaica stunned the United States with a pair of first-half goals, one off a blunder by goalkeeper Brad Guzan, and held on for a 2-1 victory in the Gold Cup semifinals Wednesday night that dealt the Americans their biggest upset defeat.

Darren Mattocks, who plays for Vancouver in Major League Soccer, put the Reggae Boyz ahead with 31st-minute header directly off a throw-in. Giles Barnes followed five minutes later with a goal on an 18-yard free kick after Guzan was caught going outside the penalty area on a routine throw.

Michael Bradley scored in the 48th minute for the Americans, but it wasn’t enough to prevent a stunning setback in front of sold-out crowd at the Georgia Dome. The small contingent of green-and-gold-clad Jamaican fans saluted their underdog team, ranked 76th in the world but now becoming the first Caribbean nation to reach a Gold Cup final. The Reggae Boyz face Mexico or Panama on Sunday in Philadelphia.

The 34th-ranked Americans, who had played in five straight Gold Cup finals, will face the loser of Wednesday’s second semifinal in the third-place game on Saturday. They also will meet the Gold Cup winner in a playoff for the region’s berth in the 2017 Confederations Cup.

It marked the first time the U.S. was eliminated by a CONCACAF team en route to the Gold Cup final. In the era when teams outside the region were invited guests, the Americans lost semifinals to Brazil in 1996 and 2003, and a quarterfinal to Colombia in 2000.

In the early going, it looked as though the Americans might romp to another impressive win after a 6-0 blowout of Cuba in the quarterfinals. They had most of the chances but kept sending good looks wide or over the net against Jamaican goalkeeper Ryan Thompson, who plays for the Pittsburgh Riverhounds in the third-tier United Soccer League.

Suddenly, Jamaica jumped ahead. Kemar Lawrence got everything on a long throw-in, delivering it perfectly into the box. Mattocks, with his back to the goal and sandwiched between defenders Ventura Alvarado and John Brooks, leaped up for a dazzling header that caught the underside of the crossbar, out of a leaping Guzan’s reach, and dropped beyond the goal line. Guzan had taken a step off his line and scrambled back for the ball, but it was too late. He slammed it in disgust as the Jamaicans celebrated.

The U.S. goalkeeper was really steaming minutes later, when his huge blunder set up Jamaica for a commanding lead.

On a routine throw downfield from the edge of the penalty area, Guzan’s right arm went over the line when he let go of the ball. That gave the Jamaica a dangerous free kick and Barnes hooked a shot over the defensive wall and into the right side of net, while Guzan was covering the opposite side.

After the goal, Guzan screamed at the linesman who made the call, but the replay showed it was the proper one.

The Americans fought back early in the second half. Aron Johannsson ripped a shot that was smothered by Thompson, but he couldn’t hang on to the ball. Dempsey tried unsuccessfully to poke it under the sprawled-out keeper, and Bradley swooped in on the third whack for the goal that sent the sell-out Georgia Dome crowd of some 68,000 into a frenzy.

Bradley nearly evened it in the 57th, when his shot one-hopped off Thompson’s chest, caught the near post and deflected away.

The Americans had a number of good chances the rest of the way, but none that came close.