Couple of important points here from the Fatigue class I attended last year headed by DW (ALPA safety chair).

You can't preemptively not elect to extend. The key time is after you have exceeded your duty day without the extension. I know we're trying to save the day (operation) but you have to look out for yourselves and other pilot in this case.

Second point is the phrase is "I'm/we're not fit to continue". Using "fatigue" brings a whole different level of complexity.