Winger Bryan Habana equalled Jonah Lomu's record of 15 Rugby World Cup tries with a hat-trick for South Africa, who qualified for the quarter-finals as Pool B winners after a 64-0 demolition of the United States.
The 1995 and 2007 champions have won three on the bounce since their shock opening defeat against Japan and will face the Pool A runners-up, either Wales or Australia, at Twickenham on 17 October.
Habana's hat-trick came in the first 20 minutes of the second half, with the rampant Springboks crossing 10 times in all and flanker Francois Louw scoring two tries of his own.
South Africa have 16 points from their four matches and cannot be overhauled at the top of the group. Scotland or Japan will go through as runners-up.
The Springboks stormed ahead after seven minutes as, either side of missed penalties from the United States, De Allende cantered through the USA defence before sprinting over for his first international try.
Handre Pollard converted before South Africa-born scrum-half Niku Kruger came close to pulling the United States to within four points - only to see his penalty come back off the post.
The Americans, with 12 changes from the defeat to Scotland, were dominating possession but could not make it count as Blaine Scully and Habana were both forced off for head injury assessments after the South Africa winger's strong challenge.
Pollard broke free and aimed for the corner but the United States defence held firm as both Kruger and Zach Test did well to prevent Kriel scoring the second try - with the television match official ruling against the Springboks.
The USA were penalised after their scrum twice collapsed, with referee Pascal Gauzere awarding a penalty try to South Africa - with Pollard again converting.
Scully, back on having been cleared of any head injury, led a break-out close to half-time but opted to check inside rather than test his pace down the flank and he was soon swallowed up by the South Africans.
The third try came just a minute into the second half as Habana beat a marginal offside call to collect a pass from skipper Fourie du Preez for the 13th World Cup try of his career.
Hooker Du Plessis added the fourth as the USA began to toil, and Louw was next to go over to add to his try from the defeat against Japan.
The 32-year-old Habana ran over his second before matching Lomu's haul of 15 World Cup tries with a simple hat-trick score moments later.
The record should have been in the bag with 13 minutes remaining but Habana fumbled when in behind - only for Louw to double up for the evening and Kriel to then push the score along even further.
Mvovo added the final try with Morne Steyn converting as the United States became the first side not to register a point in a match at this World Cup.