Key fast bowler Steyn has played no part in the tournament to date after suffering a side strain in the warm-up defeat to Pakistan.
But the 29-year-old appears ready to be unleashed for the winner-takes-all battle for a semi-final berth.
Captain AB de Villiers said: "He bowled at a hundred per cent intensity yesterday, and it was sort of the first part of his fitness assessment and today he's doing a bit more. He's 99.9 per cent ready, but we're just going to make sure this afternoon that he'll be ready to go.
"It's almost a yes, just not yet. We have just got to wait a few hours and then we'll make the final call this afternoon, but it's looking good."
The weather looks certain to have some sort of say in Cardiff, with sporadic showers ensuring the middle was under cover for much of Thursday.
A shortened game would appear to play into the hands of West Indies, given their station as World Twenty20 champions, although the Proteas hold the advantage in terms of net run-rate should the conditions prevent a result. But De Villiers is confident his side will be able to cope with whatever is thrown at them.
"Whether we play a shortened game or a normal game, it's all about focus and team intensity off the field," he said. "I mention this so often, but it's really true, the guys that can adapt the quickest can really walk on the field and switch on straight away. No matter what happens, the rain is out of our control, so we'll focus on good cricket whether it's a 20-over game or a 50-over game.
Despite their undoubted quality, South Africa have not won an ICC event since 1998, enduring a number of near misses, particularly at World Cups. And West Indies skipper Dwayne Bravo believes the tag of being "chokers" puts all the pressure on Gary Kirsten's team, despite the Windies' last ODI win over the Proteas coming in 2006.
He said: "I think in tournaments, it doesn't really matter whether you win previous games against opposition or not. I think the South Africans will feel the pressure also. They know if they lose, they are going back home. They have a tag of being chokers that do not do well in big tournaments, so that would be added pressure on them.