it is just davis cup, who cares?

For Joker, Playing for davis cup is purely from diplomatic point of view. He showed to his country: look, it is not my fault that Serbia can not go to the final,  I tried my best already.

Actually, Joker is very smart, when he senses that he can not win, he will just retire so that he can avoid injury and save energy for future tournament. He said after his retirement from Cincinnati Master: Yes, I can still continue to play a few more games, but what for? I can not beat Andy Murray with one shot.

Yes, barring injuries, I still think Joker has a very good chance to win the Calendar Grand Slam next year.