Access to the Singapore club is not based on SkyClub membership. It is open to BE pax and ST Elite+ from 3-6 am, which is prior to the departure of the DL SIN-NRT flight; I believe you have to show a ticket for the flight to use it. But SIN has so many great places to eat or visit that the lounge doesn't really matter; visit the butterfly garden, take a swim, or eat some Singapore food.

This is a useful site:

http://www.delta.com/traveling_check...ions/index.jsp