South Africa captain Aiden Markram warned his side against under-estimating the United States ahead of their T20 World Cup showdown, claiming the Americans are "not a small team anymore".

Co-hosts USA stunned the sport by beating former champions and Asian powerhouse Pakistan in the first-round group stage, a result which helped propel the Americans into the elite Super Eights second stage.

"They have been really good. A lot of people will say 'smaller nation', but they're not and they've proved that they're not anymore," said Markram.

"So, we're going to 100 per cent have to be on our best game for things to go well, but I'm excited for that challenge."

With 12 of the original 20 nations eliminated, the United States are riding a wave of confidence into the Super Eights.

Two-time winners West Indies and defending champions England complete their second round group.

India, Australia, Afghanistan and Bangladesh comprise the other pool.

South Africa finished their opening group campaign with a perfect four wins in four games but they rode their luck to maintain a 100 per cent record.

After bowling out Sri Lanka for just 77 and claiming a six-wicket victory, the Proteas edged the Netherlands by four wickets in pursuit of just 104. At one stage, they were teetering at 12-4.

They then squeezed past Bangladesh by only four runs before avoiding a seismic shock by breaking Nepal hearts in a knife-edge one-run win.

In common with many sides, the South Africans struggled for runs in the three games they played in New York.

However, even in the more batsman-friendly environment of the Caribbean they could only muster 115 against Nepal, their best total so far.

Only David Miller, with 101 runs over four innings, has managed to break into three figures for the tournament.

He is the lone South African in the top 20 run-getters.

Opener Quinton de Kock, his country's all-time leading scorer in the format with 2,389 runs, has only 48 at this World Cup with a best score of 20, in his four visits to the crease.

Markram has contributed only 31 runs at an average of less than eight.

"Still striving for that perfect game," admitted Markram of his team's shortcomings with the bat.

Of the bowlers, Anrich Nortje has shone with a tournament second-best nine wickets with a top return of 4-7.

"Our bowlers have been really good for us and potentially bailed us out of jail once or twice. But it's understandably so," added Markram.

"Conditions have been quite tough for us with the bat. Now it's a fresh start for us. Specifically, if you speak as a batting group, we get the chance to get new conditions, hopefully slightly more batting friendly for us."

The USA team have a South African in their line-up in left-arm fast-bowling all-rounder Shadley van Schalkwyk.

The Cape Town native studied at the same school as former Proteas star Jacques Kallis.

Van Schalkwyk, 35, has become hard to miss at the World Cup as he superstitiously wears the same red, white and blue socks for every match.

"I think he has like maybe four or five pairs of those socks to be honest with you," said US batsman and stand-in skipper Aaron Jones.

"He really enjoys it. We like it. Every game we look forward to see which socks he's going to wear."