Both players missed Wednesday's warmup against Pakistan and Cricket South Africa said in a statement that a final decision on their availability will be made on the day of the Sri Lanka match.
CSA said Steyn will continue to receive treatment to give him the "best chance of recovery for the opening match" but would be wary of rushing du Plessis because of the hectic schedule.
AB de Villiers is expected to lead South Africa if du Plessis is unavailable.
The World T20 Super 10 stage commences Friday with India versus Pakistan in Mirpur.
