GB have been drawn against Serbia in the last 8 in Malaga (21-26 November).
Djokovic, who won with Serbia in 2010, says the competition is his top priority for the rest of the season.
But the ties will be the best-of-three matches, and Serbia do not have any other singles players in the top 30.
"We've topped this group so we expect to get a kinder draw in the quarter-finals but it's not panned out that way," Great Britain captain Leon Smith told BBC Sport.
"But you know what, we deserve to be there - and we've got a chance against anyone. It doesn't matter if Novak plays for Serbia, or anything else. You've got to win two [singles] rubbers and a doubles and that's what we've proved to be good at."
https://www.bbc.co.uk/sport/tennis/66852363