Cape Town - The Blitzboks topped Pool A after making it three wins out of three by beating the United States 21-15 at the Cape Town Sevens on Saturday.
After a period of sustained attack from the kick-off, the Blitzboks opened the scoring in the third minute when Dylan Sage ghosted through a gap to score under the uprights.
His try was converted by Justin Geduld as South Africa took a 7-0 lead.
From the restart, however, South Africa were down to six men after Chris Dry received a yellow card for a dangerous tackle.
The United States could not make their numerical advantage count as the Blitzboks extended their lead to 14-0 after Rosko Specman collected a Geduld kick upfield to score a converted try under the posts.
The United States hit back immediately through the pacy Carlin Isles who received the ball out wide and cruised away from the Blitzbok defence to cross the whitewash for an uncoverted try.
The Blitzboks were again down to six men as Seabelo Senatla was yellow carded for a tip tackle in the move that formed part of Isles' try.
At half-time South Africa led 14-5.
With both defences in top form, it took four minutes for the opening score of the second half as Specman scored his second of the match to extend the Blitboks' lead to 21-5.
But the United States were not out of it yet as Matai Leuta crashed over in the left-hand corner for an uncoverted try that saw South Africa's lead cut to 21-10.
And the United States scored again as Perry Baker squeezed over in the corner after the Blitzboks dealt poorly with a kick-off but ulitmately, they had run out of time as the hosts held on for a 21-15 win.
South Africa will now face Kenya in a quarter-final matchup on Sunday.
Kick-off is at 11:18.
Earlier on Saturday, the Blitzboks beat Fiji 26-14, outscoring their opponents by four tries to two.