Kei was asked to play, but he had scheduling conflicts. He actually withdrew from the Tokyo Open two days ago, so he most likely wouldn't play had he participated.
Like @icedevil0289 said, 3 players are chosen based on their rankings after the French Open, and the other 3 are captain's picks. I don't know the process for choosing the other 3 players. It seems for Team World, Sock, Isner, and Kyrgios will most likely be there again next year. I'm not certain about Fritz. Sure, he was a good part of the team, but there's truly a lack of diversity on that team. I mean, they had 3 Americans, 2 Canadians, and 2 Australians that is supposed to represent the world. All English speaking countries. I'd like for McEnroe to pick other guys.
the lack of diversity was really one of the main things that bothered me about team world, it was basically n.america featuring nick. I feel like nick will somehow always be attached to this event, and i dont mind it, probably because Australia is a major backer of this tournament. but i agree imo fritz could have been replaced by diego and did they really need two canadians who actually, sorry to the fans, were probably the most useless. ffs even sock won a singles match, though that might be due to fabio.
to be honest i would definitely love more clarity into the choosing process, who has control over it. I am going to assume nick (due to the promotional video for boston). Assuming they do stick with nick, john, and jack as their constants, who else would do you pick for team world?