SQ does not assign economy seats in advance...except for online bookings

From SQ.com

Economy Class customers are able to make specific seat requests on all our flights (except for flights between Singapore and Kuala Lumpur) for tickets purchased online. Customers who book their flights via travel agents or SIA offices may request for window, aisle, or emergency exit seats at point of booking. Certain conditions apply for allocation of the emergency exit seats.

Thai should assign you a seat.