The USA, making the semifinals for only the second time in World Cup history, finished with 13 points from five matches in Group A.
Defending champions Argentina were pushed to the second spot with 11 points.
Argentina, seeking their third title in four World Cups, will meet reigning Olympic champions The Netherlands in the other semifinal.
Argentina were held to a 1-1 draw by China in the last league fixture. The Chinese defence frustrated Argentina, whose strikers made repeated forays into the scoring zone only to falter.
More From This Section
USA had to wait until the 35th minute to open the account with the first goal coming seconds before half-time through Caroline Nichols.
Captain Marsha Cox equalised for South Africa in the first minute of the second session, but the USA surged ahead with goals from Katie Reinprecht (39th minute) and captain Lauren Crandall (45th).
Lisa-Marie Deetlefs pulsed a goal back for South Africa in the 47th minute before Rachel Dawson sealed the game for USA with a penalty corner conversion in the 56th.