Roger Federer was paying attention right along with everybody else when Serena Williams played what is expected to have been her last match three weeks ago at the US Open.

"Not surprised. Just very similar to me, in many ways. We were expecting it to come at some point," Federer said.

"You don't ever want players like Serena to ever retire. … I just thought, 'What a great career.'"

He recognises that their back-to-back exits after about a quarter-century each in tennis — he is 41 and leaves with 20 grand slam titles and she turns 41 on Monday and has 23 major singles championships — will spur some fans to move on from the sport.

Federer insists, though, that plenty will stick around.

"It's going to leave some fans with not the same taste for the game," Federer said in an interview at the Laver Cup, where the final match of his stellar career will be in doubles alongside rival Rafael Nadal for Team Europe about 5am AEST on Saturday.

"We might lose some, because they say, 'OK, well that chapter for me closes, and I'll move on to another sport or another athlete.'

"And some will stay with the game forever, because tennis is just a sport [that], once you're in it, you're normally in it. That's why I don't believe a lot of people will leave."

Tweeting from the Thames in London, Federer wrote: "Heading to dinner with some friends", taking a selfie alongside Rafael Nadal, Novak Djokovic and Andy Murray ahead of the Laver Cup.

It was a stark reminder of the tennis talent that is on the way out.

Federer and Williams — along with Nadal, a 36-year-old who has a men's record 22 slam titles, and Djokovic, a 35-year-old with 21 titles — helped create a golden era in the sport, drawing new viewers and inspiring new players.

"Serena and Roger probably have more fans than anybody in tennis," said Taylor Fritz, a 24-year-old who is the highest-ranked American man, at number 12, and a part of Team World in London.

"It's tough to replace two people as iconic as them, but I still think there is a lot of exciting times ahead."

Still, what happens after they leave?

"These two players are irreplaceable. I don't think there is any question about that," said John McEnroe, who won seven major trophies from 1979 to 1984.

"[But] the sport goes on, no matter what, and we have witnessed that in every sport over time."

Which is why Federer is among those optimistic about the future.

"I'm a big believer, always, that tennis is bigger than anybody," he said. 

"And it will always create new superstars."

Federer thinks highlight-reel shots that make their way around on social media can help. So can the telling of each new top player's "captivating story".

McEnroe mentioned the sport's need to do better marketing of fresh faces.

Someone Federer believes could fit the "superstar" category is Carlos Alcaraz, the 19-year-old from Spain who won the US Open and became the youngest man at number one in the history of the ATP computerised rankings, which date to 1973.

Federer watched — on TV — some of Alcaraz's epic quarterfinal in Flushing Meadows against Jannik Sinner, a 21-year-old from Italy.

It lasted 5 hours, 15 minutes, ending at 2:50am in New York — that's 8:50am in Federer's home country of Switzerland, so the father of two sets of twins missed the fifth set, he explained, "because I had to bring the kids to school".

He came away impressed by both players.

"Super movers. Great ball-strikers, forehand and backhand," Federer said.