The Americans ran in four tries to beat South Africa 22-5 (12-5 half-time) in their Plate semi-final match on Sunday. They also beat the South Africans 19-12 in the group stages on Saturday.
Earlier on Sunday, South Africa lost 19-14 to England in the Cup quarter-finals, with the hosts scoring in extra time to advance.
Results from the London Sevens
America will no play Fiji in the Plate final, after the Fijians defeated Argentina 26-12.
New Zealand won the tournament when they thumped Australia 47-12 in the final.
Scorers:
South Africa
Tries: Stephan Dippenaar
America
Tries: Zach Test (2), Shalom Suniula, Matt Hawkins
Conversions: Suniula
Results on Sunday in the London Sevens at Twickenham, the last leg of the Sevens World Series:
Cup
Quarter-finals
New Zealand 52, Argentina 7
Kenya 20, Fiji 7
Australia 24, United States 19
England 19, South Africa 14, ET
Semi-finalsNew Zealand 7, Kenya 0
Australia 14, England 7
Third place playoffEngland 26, Kenya 19
FinalNew Zealand 47, Australia 12
PlateSemi-finals
Fiji 26, Argentina 12
United States 22, South Africa 5
FinalFiji 14, United States 5
BowlSemi-finals
Wales 21, Samoa 12
France 28, Canada 14
FinalWales 19, France 7
Final series standings:
1. New Zealand, 173 points
2. South Africa, 132
3. Fiji, 121
4. Samoa, 104
5. Kenya, 99
6. England, 92
7. Wales, 91
8. Australia, 89
9. France, 87
10. Argentina, 84