It was the Blitzboks' second defeat of the day as the team struggled to replicate their sublime form from Saturday, going down to Samoa with a golden point in the semi-final before struggling to put away the Americans.
USA had all the early possession and it wasn't much of a surprise when Marcus Tupuola exploited space out wide for the first try in the third minute, and shortly thereafter their lead stretched to 10-0 when Malacchi Esdale scored their second five-pointer.
The Blitzboks pulled one back when Siviwe Soyizwapi raced away from a clever cross-kick by Dalvon Blood, and Du Preez's conversion made it 10-7 with just under two minutes to go in the half.
But with time up on the clock, USA forced a turn-over at a ruck and Joe Schroeder ran all of 50m to score their third try for a 15-7 lead at the break.
A minute into the second half, the Americans stretched their lead to 15 points when Faitala Talapusi scored their fourth try, leaving a mountain to climb for the Blitzboks if they were to win bronze.
Blood closed the gap with a great try down the blindside and with Ricardo Duarttee adding the extras, it was 22-14 with three minutes to go.
They kept on making uncharacteristic mistakes in their own half, and the USA held on for the win and the bronze medal.
Springbok Sevens 14 (7) - Tries: Siviwe Soyizwapi, Dalvon Blood. Conversions: Branco du Preez, Ricardo Duarttee.
USA 22 (15) - Tries: Marcus Tupuola, Malacchi Esdale, Joe Schroeder, Faitala Talapusi. Conversion: Tupuola.
Earler on Sunday, the Springbok Sevens team were knocked out of the Cup competition at the HSBC Cape Town Sevens in the cruellest of ways as Samoa beat them by 10-7 with a golden penalty goal in extra time of their semi-final at the DHL Stadium.
Scores were level at 7-7 at the end of regulation time, setting up extra time and the golden point rule, which stipulates that the first team to score will be deemed winners.
Samoa restarted, the Blitzboks knocked on and the referee awarded a very technical penalty to the men from the South Sea Islands, which Faafoi Falaniko calmly steered between the uprights from 40m out deny South Africa a second successive tournament final, and set up a decider against New Zealand on Sunday evening.
Before that, there was very little to separate the two teams.
Ricardo Duarttee was yellow carded for a synical infringement two minutes into the game with Samoa hot on the attack, and they capitalised soon thereafter with a converted try by Vaovasa Afa Sua.
The Blitzboks didn't get the rub of the green from the referee at the breakdowns, which kept them on the backfoot for large parts of the first half, and neither side managed to score again, with Samoa leading by 7-0 at the break.
Samoa's try scorer was yellow carded with four minutes to go after repeated infringements and from the ensuing penalty, Darren Adonis raced away only to lose the ball in a tackle with the try-line begging.
Ryan Oosthuizen capitalised by crashing over from a penalty shortly thereafter, with Duarttee levelling the scores with the conversion with 90 seconds left on the clock.
The score stayed 7-7 until the end, setting up extra time and Falaniko then did what was necessary for Samoa.
Springbok Sevens 7 (0) - Try: Ryan Oosthuizen. Conversion: Ricardo Duarttee.
Samoa 10 (7) - Try: Vaovasa Afa Sua. Converion: Paul Scanlan. Penalty goal: Faafoi Falaniko.