Novak Djokovic has raised concern over Davis Cup when he said he is not sure how the international tournament will survive without the legendary Roger Federer.

While Djokovic has agreed to represent Serbia in the Davis Cup, starting next month in Madrid, Federer has made himself unavailable for Switzerland. Germany's upcoming talent, Alexander Zverev, has also withdrawn from the event, which has been totally revamped by new owner Gerard Pique, the star FC Barcelona defender.

Speaking at the Shanghai Masters, Djokovic confirmed his participation at the Davis Cup, as he said, "With Davis Cup, you know, Roger is not playing and, you know, some other guys, so how that’s going to turn out, I don’t know."

"I’m going to play it this year. Whether that’s going to be the tradition, so to say, every year, I don’t know, but the week is really tough, I mean, especially for top guys playing all season and then, you know, playing in London and then literally the next day you have to take a flight and potentially play the next day on different conditions and everything," the World No. 1 added.

The Serbian also insisted that it would be better if Davis Cup merged with the ATP Cup, which is due to host its edition next year in January.

Novak Djokovic and Roger Federer
In this picture, Federer of Switzerland (right) and Djokovic of Serbia react during their Men's Doubles match on day one of the 2018 Laver Cup at the United Center in Chicago, Illinois, Sept. 21, 2018. Matthew Stockman/Getty Images for The Laver Cup

He also added that having two separate events, that too just six weeks apart will not last in the long run.

"I don't think they can both coexist six weeks apart for many, many years. Formats are very, very similar if not the same. Of course, Davis Cup has the history behind it and tradition and of course the credibility and popularity of a hundred years. ATP Cup is a completely new event, but ATP Cup has a better week. It's the beginning of the season a few weeks before the Australian Open starts. Most of the players are already on Australian soil so most of the top players are gonna play there," Djokovic said.

"I think for the sake of tennis, I honestly hope that the conversations are going to happen again on merging two events into one, a super event, because I think it’s necessary, honestly," Djokovic further explained.