Manteo Mitchell ran the opening leg for his country but felt his left fibula break at around 200 metres.
He refused to give in to the injury, though, and managed to complete his lap in 46.1 seconds before handing the baton over to his teammate. USA finished with a time of 2:58.87 - the joint fastest in the qualifiers.
Mitchell said that the break was caused by an awkward slip on the stairs a few days earlier, but got treatment and was feeling fit for the race.
Recalling the race, the American stated: "I got out pretty slow, but I picked it up and when I got to the 100-metre mark it felt weird. I was thinking I just didn't feel right.
"As soon as I took the first step past the 200-metre mark, I felt it break. I heard it. I even put out a little war cry, but the crowd was so loud you couldn't hear it. I wanted to just lie down. It felt like somebody literally just snapped my leg in half."
Mitchell explained: "I knew if I finished strong we could still get [the baton] around. I saw [teammate] Josh Mance motioning me in for me to hand it off to him, which lifted me.
"I didn't want to let those three guys down or the team down, so I just ran on it. It hurt so bad."
Max Siegel, the USA Track and Field chief executive, was quick to praise Mitchell, stating: "Manteo has become an inspiration and a hero for his teammates.
"Without his courage and determination to finish, Team USA would not be at the starting line in the final. The team has rallied around him and we are all looking forward the final days of competition."
An x-ray after the heat confirmed the extent of Mitchell's injury. Team USA will announce Mitchell's replacement for the final.
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