If you have a valid visa to SIN, you can enter the country from any place - no problem, either from Thailand or from Switzerland The must things to have at hand: onward ticket, prebooked accommodation, proof of sufficient funds. ALL these requirements may be asked from you by the immigration in SIN, the failure to provide one may cause your entry denial.
Dosiano, you say that you only have a single entry visa, so on that basis you can only enter the country once.
At that point, if you try and enter Singapore the second time on your return journey, having entered once already on the way out, your single entry visa will not be valid as you will have used it.
Now Changi airport has a full transit system for full serrvice airlines, so you can route back via Singapore airport so long as you are only in transit and staying airside and do not enter Singapore (ie go through immigration) itself.
Any full service airline can do that but the low cost carriers that operate point ot point mostly require you to disembark and immigrate to check for the onward flight....that would not work for you since you do not have a valid entry visa so you need to avoid those airlines that mean you have to do this process.
So while it is possible for you to route back via SIngapore you need to be careful with the airlines you use and a single entry visa would not be valid on the way back for entering Singapore for a stopover.
You could transit only on the way out to Thailand and then use the visa to enter on the way back as an alternative assuming the dates are still valid (check that carefully).
So this all comes down to which airlines you are flying with, and how and when you intend to actually go through immigration with the visa that you have....Edited: 09 October 2013, 16:07
If you are ALREADY in SIN now and used your single visa- the answer is NO, you can not use it again and can not re-enter SIN with it.
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