The New Zealand side earlier outplayed France to advance to the final four, where Fiji and Australia will meet in the other semi-final at Dignity Health Sports Park.
It was a win that demanded courage and perseverance against a home side cheered on by the local crowd, especially as USA speedster Carlin Islis rounded the Blitzboks in the opening minute to edge his side ahead.
Isles was on the scoreboard again four minutes later, dotting down in the corner after USA won a good turn-over ball.
The Blitzboks came right back just before the break when they worked some numbers on the left and Ryan Oosthuizen scored his 100th point for the Blitzboks.
Trailing 10-5 at the break, the second half became a tense affair. Werner Kok was held up over the line as the Blitzboks started to put severe pressure on the USA defence and JC Pretorius bounced off a couple of defenders to score under the sticks with a minute to go. Selvyn Davids converted to give his team the lead.
It was a proper battle, the Blitzboks clinching the win with a minute to go through a try by JC Pretorius. 12-10 the score, now @AllBlacks7s in the semi of the @LASevensRugby @CastleFreeSA @FNBSA @ASICS_ZA pic.twitter.com/PfA2OCFomW— Springbok Sevens (@Blitzboks) March 1, 2020
The USA had one more attack up their sleeve, but clever defence by Kurt-Lee Arendse - who slapped a USA pass towards his own line - resulted in possession lost by the home side. Stedman Gans collected the bouncing ball and kicked it into the stands.
South Africa 12 (5), USA 10 (10)
South Africa - Tries: Ryan Oosthuizen, JC Pretorius. Conversion: Selvyn Davids.
USA - Tries: Carlin Isles (2).