Andre Dirrell upset Cuba's Yordani Despaigne 12-11 in the middleweight quarter-finals to clinch a second boxing medal for the USA.
And Thailand's Suriya Prasathinphimai stunned Ukraine's Oleg Mashkin, runner-up in the world championships, 28-22.
Prasathinphimai now faces Russia's Gaydarbek Gaydarbekov who defeated Cameroon's Hassan Ndam Njikam 26-13.
Dirrell will fight Gennadiy Golovkin on Friday after the Kazakh beat Egypt's Ramadan Yasser 31-20.
Dirrell hung back in the early rounds before ambushing Despaigne and scoring five points in the fourth round to hang on for a one-point victory.
Despaigne was just the third Cuban fighter to fail to clinch a medal.
Despite Dirrell's victory, the USA will finish with its worst boxing medal haul since 1948.