John McEnroe says Roger Federer and Serena Williams are "irreplaceable" but that tennis "goes on no matter what" as the Swiss prepares to play his final match on Friday.

Federer, a 20-time Grand Slam singles champion, will bring the curtain down on his glittering career in the Laver Cup at London's O2 arena alongside long-time rival Rafael Nadal.

The two players, representing Team Europe, will take on Team World's Frances Tiafoe and Jack Sock in doubles in the evening session on the first day of the event.

The 41-year-old Federer's retirement comes hot on the heels of a similar decision by Williams, who won 23 majors in singles.

McEnroe, who is captain of Team World, admitted the sport would miss the two players but is optimistic about the future, just weeks after 19-year-old Spaniard Carlos Alcaraz won the US Open.

"These two players are irreplaceable," he said. "I don't think there is any question about that."

But he added: "The sport goes on no matter what, and we have witnessed that in every sport over time.

"The opportunity is there to market these young kids in a way that I don't think we have successfully done before."

McEnroe, who won seven Grand Slam singles titles, said Williams had "brought electricity" to the recent US Open, where she lost in the third round.

But he said other players such as Alcaraz and America's Tiafoe, who lost to the Spaniard in the semi-final at Flushing Meadows, made it an "electrifying event".

Taylor Fritz, who is also part of Team World, said the two big retirements would be tough on a lot of fans but the future was bright.

"Serena and Roger probably have more fans than anybody in tennis," said the American. "But, you know, there's so many more players.

"I think tennis is becoming so much more entertaining because you're seeing so many new people win.

"It's really exciting that going into these big events there are 10 different people that could potentially win. Hopefully their fans stay in the sport, want to keep watching."

Canadian star Felix Auger-Aliassime believes there are exciting times ahead with "a lot of new players playing very good, very intense and entertaining tennis".

But he hopes Federer and Williams stay around the game.

"They have been icons, idols to all of us sitting here," he said at a pre-tournament press conference. "Hopefully, you know, they have a chance to give back in some form and stay around tennis."

Nadal, 36, who first played against Federer on the ATP Tour in 2004, said he did not need the retirement of his long-time adversary to remind him that "the end is closer".

"You know, the normal cycle of life is this," he said. "Some people leave and others need to come.

"Nothing new. History repeats always. Just this time it's us, and in this particular case is probably one of the most if not the most important player in the history of this sport that is leaving after a super great and super long career."

Andy Murray, another veteran member of Team Europe, admitted thoughts of retirement had crossed his mind.

"As you get older as athletes and with some of the physical issues you do think about what if or when should you stop and when is the right moment and how would you like it to be," said the three-time Grand Slam champion.

"I have thought about it myself, but I don't think there's many better ways to go out than like this."