UA opens quite early because of the earlier flight to HKG. I usually get to the airport at around 5:00 am in Terminal 1. (travelled every single week for 3 months!) Already a lot of people there. I asked the GA about the openning time. He said about 4.
If you are coming in from Macau on Tiger Air, you may be using the new budget terminal. The best alternative at that time would be to take a cab to the nearby Geylang district for food and sightseeing. Then take the cab or MRT back to Terminal 1 for checking in. The SQ lounge in Terminal 1 is lousy anyway. Skip it. Just use other better paid restaurants there. Even McDonald is a lot better than that lousy lounge.