Key fast bowler Steyn has played no part in the tournament to date after suffering a side strain in the warm-up loss to Pakistan.
But Steyn, 29, appears ready to be unleashed for the winner-takes-all battle for a semi-final berth.
"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," captain AB de Villiers said.
"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 straightaway.
"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.
"We still rate ourselves as a serious cricket team in whatever format we play and however many overs we play in. I feel we've got a bit of momentum behind us at the moment. We play really good cricket, and that intensity I often talk about is really spot on at the moment.
"Whatever happens tomorrow, we'll go out and make sure we play better than the opposition."
Despite their undoubted quality, South Africa have not won an ICC event since 1998, enduring a number of near-misses, particularly at World Cups.
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.
"I think in tournaments, it doesn't really matter whether you win previous games against opposition or not," Bravo said.
"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. For us, we're going to enjoy the occasion and forget about what happens in previous games.
"It's a tournament. It's a knockout game. We've been in this situation before and do well and go beyond.
"So, no pressure is on us. I'm not worried about the record of the previous games. I'm really excited and looking forward to this game tomorrow."
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