The winger scored three times in 21 second-half minutes to draw level with Lomu's mark of 15 World Cup tries.
The Springboks ran in eight second-half tries in all to earn a bonus-point win at the Olympic Stadium.
South Africa top Pool B and will now play the loser of Saturday's Pool A decider between Australia and Wales.
A tale of two halves
A mixture of South Africa's ill-discipline and poor finishing kept the score down in the first half, while USA only made it into the Boks' 22 in the last minute of the half, despite enjoying plenty of possession.