Twenty-time grand slam champion Federer and Nadal, a 22-time major winner, will face Frances Tiafoe and Jack Sock of Team World at London's 02 on Friday night at around 9pm.
"I'm not sure if I can handle it all but I'll try," said Federer, 41.
"This one feels a whole lot different. I'm happy to have him on my team and not be playing against him.
"Playing with Rafa feels really different. To be able to do it one more time, I'm sure it will be wonderful and I'll try my very best.
"I'll enjoy it but it will be very hard."
Earlier this week, Federer told a packed press conference at the 02 about the idea of playing alongside Nadal, saying. "I mean, I think it could be quite a unique situation, if it were to happen, for as long as we battled together to having always had this respect for one another, the families, our coaching teams, we always got along really well.
"For us as well to go through a career that we both have had and to come out the other side being able to have a nice relationship, I think, is maybe a great message as well to not just tennis but sports and maybe even beyond.
"For that reason I think it would be great. I don't know if it's going to happen, but I think it could be obviously a special moment."
Earlier in the evening Andy Murray, making his debut at the Ryder Cup-style event, will take on Australia's Alex De Minaur.
In the day session Casper Ruud faces Sock and Stefanos Tsitsipas meets Diego Schwartzman.