One of my friends wrote the following sentence on his wechat moment(like facebook):
The problem with being competitive and running with a friend is that you may find yourself at their mercy as to when to stop.
I could not understand the meaning and structure of the sentence.
1, what is the subject of the sentence? Is it 'The problem'?
2, What does the pronoun 'their' represent?
I looked up the dictionary and found the meaning of 'at someone's mercy' is under the control of someone and without defense against someone. But I still could not understand the meaning of the whole sentence.
3, What is the actual meaning that my friend wants to express?
4, what is the structure of 'The problem with being competitive and running with a friend' in the sentence?