There are two opinions, both valid.
One is that it's a singular noun, and just because it ends in S doesn't mean we change the rules. thesis's
The other is that when singular nouns end in S, adding the apostrophe is enough since most people will say "thesis" anyway and not "thesis-iz."
I belong in the first group and I would write thesis's and say "thesisiz."
HOWEVER: my best advice on this one, given that you have a thesis and not a person with a name like "James" is to write "the quality of the thesis."
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