When you talk about someone's stamina you are literally saying that he or she is at the mercy of the Fates.
|Her stamina has definitely run out|
When "stamina" first came into English around 1700, it was as the plural of this word "stamen". Maintaining the metaphor of the thread spun by the Fates, it meant all the bodily characteristics that could be expected to determine one's life expectancy. That sense died out, leaving us with the current meaning, the capacity to endure fatigue and exertion.
The word ceased to be plural and became a singular. Knowing this may come in handy the next time someone berates you for using "media" or "data" with a singular verb. Would they say "Her stamina are extraordinary"?