In training, if you're meant to be doing 10 sets of something, do 10. Or 11. Or 12. But never do nine.
If you do less, that's the start of a sharp decline. If you don't finish a session, you might as well retire then and there, because you're done.
There will always be excuses - whether you're injured, tired, have done something tough the day before - whatever. It's all irrelevant.
I never gave any excuses, and I wouldn't expect to hear any from anyone I work with.
You should also always finish your match or competition, especially if you're injured or ill or losing badly.
I've seen so many players against me just give up. They knew they weren't going to win, so they created an illness or injury.
But out of respect for yourself as much as anything, you should go through that process, to stand there at the end having been thumped.
Shake your opponent's hand while looking into their eyes. Finish the match, whatever the circumstances.