Australia have been toppled from the world top spot for the first time since global rankings began in 2002 after losing by five wickets in a record chase by New Zealand on Sunday, following a 10-wicket thrashing on Friday.
South Africa assume the mantle of the world's top-ranked side, and while Australia appear in a serious slump, having lost their last four games and two series, Fleming said they could not be counted out.
"Maybe Australia might be feeling the pinch in terms of getting results, but they're never going to (be) far away either," he said.
"I think that's where international cricket is at the moment. We're going into a World Cup where on any given day there's up to eight teams that can win it."
While Australia's stocks are diving, South Africa are brimming with confidence, having won seven of their last eight one-day matches.
Even if Australia win the final match of the Chappell-Hadlee series against New Zealand in Hamilton on Tuesday, it will not be enough to regain the top spot before the World Cup starts.
South Africa have 128 points, while Australia have 126 and Pakistan are third, less than a point ahead of New Zealand.
Australia's stand-in captain Mike Hussey said his side's loss at home to England in the finals of the recent tri-series and then losing away to New Zealand had steeled their resolve as they seek a third consecutive World Cup.
"Now we become the hunter," he said.
"We'll be working as hard as we always do to get that number one spot back."
Australia's prospects of avoiding a 3-0 drubbing by New Zealand have been bolstered by the unavailability of Shane Bond and Daniel Vettori for the dead rubber on Tuesday.
New Zealand's two most experienced bowlers suffered minor injuries on Sunday -- Bond had a tight hamstring after bowling nine overs while Vettori's troublesome back is causing more discomfort.
Neither problem is considered more than an irritant and had the series been on the line, the duo would likely have played.
Bond has proved almost unplayable by the Australians, taking 5-23 in the first match, and 1-7 in his first six overs of the second match.
He has been replaced by left-armer James Franklin while Vettori makes way for Jeetan Patel.
Hussey is keen to win the final encounter to salvage something from the New Zealand tour.
His captaincy record is now three losses from three games, despite having twice scored centuries in leading losing causes, which has prompted him to speculate on a selfless solution.
"Maybe I should get a duck, then we might win," he said.