World champions Australia slipped to third place in the one-day rankings on Wednesday, their lowest since the International Cricket Council (ICC) introduced ratings in October 2002.
On Monday Australia collapsed to their seventh defeat in nine matches against top-ranked South Africa this year and now lie third in the rankings behind India.
However, Australia will climb back to second if they win the last match in Johannesburg on Friday.
South Africa won Monday’s fourth game by 61 runs to clinch the five-match series 3-1 with a match to spare.
Australia held the top spot for all but 10 months of the duration, including 52 straight months from October 2002 to January 2007, ICC said in a statement.