Playing from another country/continent
you shouldn’t encounter any issues as long as you authenticate your log in attempts via your e-mail or authenticator if enabled.
ArenaNet Communications Manager
Since you purchased and registered your game in the US, you should be just fine playing from Singapore in an access sense. I cannot speak to the latency from Singapore, but we have a lot of players in Australia and New Zealand playing on US servers, and they report things are going well. So I would think that Singapore, at a somewhat similar distance, would also be playable. (The quality of your Internet connection will be a factor, of course.)
Communications Manager
Guild & Fansite Relations; In-Game Events
ArenaNet
I have been with this game on the same computer almsot everywhere from New York to San fransico from america to europe and even to asia with no problem ofcourse my internet was pretty good but i believe you wont have many issues