The Springbok Sevens, who beat England in their solitary victory to finish third in Pool C on the opening day, will face Russia in the bowl quarterfinals on Saturday.
Facing Samoa for a place in the cup playoffs, the Blitzboks were outplayed as they slumped to a 10-5 defeat.
The Samoans romped to a 10-0 lead at the break, and while Chris Dry scored a consolation try in the second period, the South Africans went down in their second loss of the day.
Earlier, they had hit back to beat England after falling to minnows Portugal in their opening match of the tournament.
The Blitzboks raced to a 12-5 half-time lead over England, thanks to tries from Dry and Paul Delport, and while the English clawed their way back to within two points in the second period, Stephan Dippenaar dotted down with less than two minutes left to secure a 19-10 win.
In their tournament opener, Portugal had taken a 7-0 advantage into the interval, and while Dry, who dotted down in all three pool matches, and Philip Snyman scored in the second half to put South Africa in front, Aderito Esteves went over in injury time to hand the Portuguese a surprise 12-10 victory.
Results:Pool A
Fiji 14, Spain 0
Kenya 12, Scotland 5
Fiji 22, Scotland 14
Kenya 10, Spain 7
Spain 12, Scotland 7
Fiji 14, Kenya 10
Pool B
New Zealand 14, Wales 7
Argentina 17, Russia 10
New Zealand 31, Russia 14
Wales 14, Argentina 7
Wales 22, Russia 5
New Zealand 17, Argentina 7
Pool C
South Africa 19, England 10
Samoa 24, Portugal 19
Portugal 12, South Africa 10
England 19, Samoa 17
Portugal 22, England 21
Samoa 10, South Africa 5
Pool D
Canada 20, France 0
Australia 19, United States 12
France 17, United States 14
Canada 28, Australia 28
Canada 26, United States 7
France 7, Australia 0