A heavy thunderstorm hit the Sri Lankan capital early Monday and drenched the Premadasa stadium, where the day-night international is due to begin at 2:30 pm (0900 GMT) on Monday.
A Sri Lankan official said a truncated game may be played only if the weather cleared -- a remote possibility considering rain was still in the air and more inclement weather has been forecast for the next three days.
If the match is called off, Sri Lanka and South Africa will play on Tuesday, weather permitting.
A reserve day has been kept aside for all seven matches of the series also featuring India, including the final on August 29.
A practice session for the Indian team at the Sinhalese sports club was called off on Monday as the rain showed no signs of abating.