As Roger Federer and his balky knees approach 40 years old, it looks as if Novak Djokovic and Rafael Nadal will surpass the Swiss Maestro’s once-untouchable record of 20 Grand Slam titles.

But unquestionably nobody will pass Federer as the most popular, elegant tennis player ever — with his beautiful all-court game, picture-perfect sliced backhand, touch volleys and crosscourt top-spin forehand.

According to Forbes, Federer was the world’s top earner from June 2019 to June 2020 in its annual “World’s 100 Highest-Paid Athletes.” Federer topped the field at $106.3 million.

A gentleman and the sport’s top ambassador, Federer, who turns the Big 4-0 on Aug. 8, has been a joy to watch.

And the question is for how much longer. Wimbledon is upon us Monday, and even with Nadal out of the field, it’s hard to imagine Federer traipsing to the finals with his recent résumé and knee issues.

Federer dropped out of the French Open after getting to the fourth round, presumably saving himself for Wimbledon, his favorite event.

He’s eyeing the Olympics in Japan in late July, though he has yet to commit. The singles gold medal is the only tennis honor he’s never won. Can his knees hold up for the U.S. Open, which begins Aug. 30 with a full-capacity crowd ready to serenade Federer for the final time?

“Those are questions we don’t know the answer to,’’ ESPN’s John McEnroe said on a conference call to promote Wimbledon this week. “We hope that he’s able to play as long as he wants to play. He’s sort of like our Tom Brady. You look at these older guys, they’re inspiring people like Roger certainly.’’

Federer underwent more knee surgery in 2020 and virtually skipped the pandemic tennis year. In missing the Australian Open and losing early in his lone grass-court tuneup in Halle, Germany, in the second round two weeks ago, Federer comes into Wimbledon with just eight matches in 2021.

Tennis critics noticed his bad body language toward the match’s late stages as Federer fell to Felix Auger Aliassime 4-6, 6-3, 6-2 at the Halle Open.

“I think it’s going to be an uphill battle for Roger only because, when you watch that last match that he lost, he voices sentiments that he was so disappointed he had to take a couple hours to think about it [being] disappointed in himself,’’ tennis legend Chris Evert said on the same conference call.

McEnroe and Evert each retired at 34.

“There gets a point where you wake up in the morning, you go out, even the will to win isn’t enough,’’ Evert said. “It’s just not there. Why not? People don’t understand. They think you’re going to come out there every day and play your ‘A’ game. That doesn’t happen, especially when you play 20 years on the tour. It’s going to be really monumental, to me, if he could even reach the final. It’s possible obviously. To me it’s just going to be very, very hard to do that.”

Djokovic will be the heavy favorite as he vies for the first men’s singles calendar-year Grand Slam since Rod Laver in 1969. A Wimbledon title also would tie him with Federer and Nadal for the record 20 Grand Slam titles.

If Djokovic pulls it off, then wins the Slam at the U.S. Open in September, it would be a crowning achievement. The Serbian would accomplish something Federer and Nadal never could pull off — winning each major in the same calendar year.

Still, Federer’s legacy will forever sparkle, even if he hasn’t won a major since the Aussie Open in 2018. Wimbledon has always been Federer’s most treasured spot — the grass court suiting his game best. He has eight Wimbledon titles and this could be his grass-court swan song.

“It’s no question that Roger’s best chance is here,’’ McEnroe said. “Obviously, Novak is a huge favorite. After that you would put him in the mix of the next five, six guys to make a run. It’s the same issue he’s got as Serena [Williams]. How is the body going to hold up, can he go through seven matches? That’s iffy. He looked fine in the French. He played a four-hour match, won that, then defaulted.

“But hopefully,’’ McEnroe said, “he’ll go out on his terms.’’