The time-honoured Riversdale Cup has been reduced to 54 holes after torrential rain flooded the course on Thursday.

The scheduled first round of the four-round tournament for some of the world's leading amateurs was cancelled just after 9am.

A club spokesman said 50mm of rain had fallen at the course in Melbourne's eastern suburbs when the first players turned up for the Thursday tee times, and rain continued to fall making the decision clear.

Round one tee times will be used on Friday for the first round. After Saturday's second round, the field will be cut to the top 57 males and the top 25 females for Sunday's final round.

The inaugural All Abilities Riversdale Cup will not be impacted. It is to be played over 36 holes on Saturday and Sunday.