It is nothing to do with the stamina. The point is that his train has gone. It doesn't mean that he can't be a top100 player, which is fine for other countries except for the Americans.
I did have a chance to watch that match against Nishikori and the difference in quality was significant. First of all Young lacks speed, his footwork is poor and when the game is fast, like it was vs. Nishikori, his technic lets him down. You can raise the stamina, but you can't increase speed and agility more than 20-25%. It is applicable more or less for juniors. Taking into account that he will be 25 years old in 2015, it is naïve to expect significant changes in this department. During the match it was obvious that Young is nervous, his body language spoke for himself, if I remember correctly he even didn't talk to himself as he usually did in the past. And reason for nervousness was not only because of the good opponent in general but particular that opponent is from his generation that he used to outplay in juniors. Nishikori on the other hand looked calm as it would be Djokovic or Ferrer on the other side of the court.
In my opinion, he stopped improving because of bad decisions made by his family. As it was mentioned in this forum many times before, he had to work with independent coach that could convince him to work really hard. I don’t believe that Mother/father can force her/his son to work that hard as the other person who has ambitions to succeed professionally and financially. This hard work had to be done 10 years ago, but unfortunately he was too talented that he didn’t need much to succeed in juniors. The same is happening with Tomic, but he still is relatively young and has better physical abilities than Young. Upcoming 2 years will be crucial for Tomic as well. We will see.
