41-year-old Federer announced last week that he will retire in connection with the Laver Cup, in which he has ownership interests, because his body no longer responds after not least a number of knee operations.
Federer will only play doubles with Rafael Nadal to save his scarred body as much as possible.
It will be the first time since Wimbledon last year that Federer comes to play and the status is highly unclear.
Novak Djokovic has not played since winning Wimbledon last summer.
He was not allowed to play the US Open because he had not been vaccinated against covid-19.
- It is nice to be back here in London and I am full of emotions about what will happen both on and off the track.
It's awesome to be here and it's something that happens once in a lifetime to be in a team consisting of Rafa (Nadal), Roger (Federer) and Andy (Murray), three of my biggest rivals, says Djokovic right after to have stepped off the flight according to several news agencies.
43-ranked Murray out
The fact that Murray is part of the European team gives the feeling that he received a nostalgic ticket.
The Scot is only 43rd in the ranking.
- I think people who come here to see the matches will be surprised at how good the players are, says the tournament's front name Rod Laver, 84.
There is also a Swedish interest in the tournament: Björn Borg is captain with Thomas Enqvist as assistant.
The USA is led by John McEnroe together with his brother Patrick.