England only got to the Super 8 stage via their net run-rate after big wins against Oman and Namibia, plus Scotland’s dramatic defeat by Australia.
If England win and South Africa beat West Indies later on Sunday (01:30 BST Monday) the rate will be irrelevant and England will go through in second place.
If West Indies win then it will again be used to determine England’s fate, although they remain in a healthy position.
If the game is washed out, and there is rain in the forecast for Sunday, England will be relying on the Proteas to win for them to go through.
“We’ve been there before,” Brook said. “We’ve got to concentrate on winning it first and then we’ll have a chat and see what the run-rate says.”
The USA have been one of the stories of this World Cup, having qualified from a group involving India and Pakistan in the first stage.
They have lost both of their Super 8s matches, however – first against South Africa and then West Indies.
England could have moved to the brink of qualification had they beaten the Proteas on Friday but were beaten in a last-over thriller.
The meeting with the USA will be the first between in sides at the top level.
“I haven’t actually seen them play, just a few highlights here and there of their bowlers,” Brook said.
“But we’ll do that [on Saturday], do our analysis and have a proper look to make sure that we’re ready for the game.”