American welterweight Errol Spence has been reinstated to the Olympic boxing tournament at the expense of Krishan Vikas.
Vikas was originally awarded a 13-11 win over Spence but the sport's world governing body has overturned that verdict.
The Indian fighter should have had been penalised for holding and spitting out his gumshield, the appeal found.
Spence is the only remaining United States athlete left in the boxing.
The decision in the fight between Satoshi Shimizu and Magomed Abdulhamidov has also been overturned, and the referee of that bout, Ishanguly Meretnyyazov of Turkmenistan, sent home.
Vikas was awarded a 13-11 verdict on Friday despite being dominated by Spence in the closing rounds.
The US went to London with the biggest boxing squad but the result appeared to end their interest in the men's boxing tournament, with all nine of their fighters out.
Following an American protest, AIBA's competition jury re-examined the bout and found that Vikas committed nine holding fouls, but was cautioned for only one of them by referee Lars Brovil of Denmark.
Vikas was also found to have spat out his gumshield intentionally during round two, for which he received no warning.
As a result of these findings, the AIBA jury unanimously agreed that Brovil "should have given at least two warnings to the Indian boxer", altering the score to 15-13 in favour of Spence.
Spence can now guarantee a medal if he beats Russia's Andrey Zamkovoy in his next bout on Tuesday.
Defeat would mean the US will, for the first time, fail to win a men's boxing medal at a Games for which they have entered at least one fighter.
The US hope their three female fighters can salvage some pride for a country that has won a record 48 boxing golds.