Aly Raisman's missed out on the bronze medal by the narrowest of margins in the Women's All-Round Individual competition final at North Greenwich Arena on Thursday evening.
Having helped Team USA win gold in the Women's Artistic Team event on Tuesday, Raisman, 18, finished in joint-third place but she was edged off the podium after a tie-break. Compatriot Gabrielle Douglas was the overall winner.
Raisman told JC Sport: "I feel really sad and upset, but I'm really happy for Gaby."
She revealed that she wasn't aware of the tie-break rule, which takes the lowest individual score of each gymnast into action.
Having finished second in qualification, Raisman is determined to put her heartbreak behind her when she compete in the Individual beam and floor events.
Raisman said: "I only found out I hadn't won a medal by looking at the scoreboard. When I have time to reflect I'm sure I'll have a lot to be proud of.
"I'm still an Olympic world champion and it's just an honour to be here. I have to try and stay positive for the next event and this only makes me even more determined to do well."
The American gymnast recorded a score of 15.900 in the vault in the first round of four, just behind compatriot Douglas who led Group One on 15.966.
A mark of 14.333 saw her slip down to fourth place after round two, the uneven bars.
Her routine in the balance beam failed to impress the judges who gave her 14.200 to leave her in fifth place going into the final floor exercise.
Raisman finished with a stunning floor routine, with a mark of 15.133 steering her to a final score of 59.566.
Her coach, Mihai Brestyan said: "I think maybe it's a bit harsh and controversial not to give Aly a medal. She performed well, but I think she lost it on the beam. I don't know what happened. Perhaps nerves played a part. I tried to encourage her as usually she's very calm."
"Formally we don't have a medal but actually we do."