"Unfortunately, I am experiencing some very challenging times in my personal life and thus need to prioritize quality time with my son, along with focusing on my responsibilities with the ATP World Tour and Tennis Channel,” Gimelstob said in a statement. “I will always be thankful for the opportunity to coach such an accomplished player, but more importantly such a quality person.
"… I have every confidence John’s best tennis is ahead of him and he will achieve great things the rest of his career."
Isner did not say if he has found a new coach, or if he’s looking for one.
“I am disappointed to no longer be working with Justin, but fully understand his decision and his need to spend as much time with Brandon as possible,” the American said in a statement. “I am very appreciative of all his help and know I am a much better tennis player in every way after the time we have spent together."
Isner is No. 15 in the rankings.