AFAIK, Japan only requires the passport to be valid for the period of stay of a US citizen. If you have a return ticket on SQ, then it should be proof enough for them to allow you to travel as long as the passport is valid for the return trip. Visa free entry for the purpose of tourism is only allowed for a stay of up to 90 days, so if your passport is valid for at least 91 days then it shouldn't be a problem altogether.