Roger Federer at a press conference earlier this summer. His balky knee kept him out of the U.S. Open, though many of his supporters still made the trip to Flushing Meadows. (Gary Hershorn/Associated Press)

They can spot each other by their caps, and when they intersect this week on the grounds of the Billie Jean King National Tennis Center, they sometimes share in brief commiseration.

“Yeah, we miss him.”

“How cool it would be if Roger were here.”

“Surprised he cancelled the whole season.”

They’re the members of Earth’s great flock of Roger Federer fans — all the aesthetes and other devotes — and their “RF” caps look a little melancholy nowadays. They accentuate a U.S. Open with a very present absence, one assured in July when Federer closed down his season so his knee and all else could heal, making this the first U.S. Open since 1999 without “Federer” on the main-draw big board.

You know you’re a big 17-major-title deal when people don your caps in your absence. Andreas Spintzyk from Cleveland (and Germany) wore blue. Victor Megevand, Geneva: red. Glenn Humphreys, Albany, N.Y.: light yellow (which his wife got him for Christmas). Fabiano and Vanessa Menke, Porto Alegre, Brazil: white (from among their six or seven at home, including a black-and-red). Matt Marolf from Long Island City in Queens: light blue. Fourteen-year-old Thomas Lawton from Sag Harbor, N.Y.: blue.

Lawton’s father, Glenn: white, and with particular charm.

It’s old and frayed.

The family yellow Labrador chewed it.

They all love — well, save for the retriever — a player whose rarefied statistics have included a capacity to be present, present, present, always present: 65 straight Grand Slam tournaments before this year’s French Open. By now, Roger-lessness can ache. Kate Lawton, Thomas’s mother, said of this U.S. Open, “There’s a heart piece missing for me.” Said Spintzyk, who has seen Federer repeatedly at the ATP Tour stop in Cincinnati, “There’s a big chunk missing.” Said Marolf, “I mean, I still love it. I’ll love tennis when Roger retires, regardless. But it feels like something’s missing a little bit.”

In the pursuit of the beauty for which Federer’s game is known, Megevand has seen little pleasures in, say, Juan Martin del Potro’s forehand. He has loved the U.S. Open atmosphere again, on his fourth trip here. “But you need somebody who is dancing on the court, and you don’t have that here,” Megevand said.

Some of these “RF” connoisseurs knew of Federer’s absence when they got tickets. The Lawtons had their tickets since Christmas, yet nobody felt the absence more than the Menkes from the metropolis on the southern sprig of Brazil, down near the Uruguay border.

“We love Roger,” said Fabiano Menke, an attorney and university professor. “We never saw a live tournament with Roger. Just on TV. And this would be the first tournament we’d see Roger live. And we bought the plane tickets in February this year, and, ‘OK, we’re going to see Roger.’ And I told Vanessa, ‘We must go this year. I don’t know if he plays next year. He’s been playing 16 in a row or something.’ ”

Then came the news. “We were very sad. . . . We didn’t say we were coming to the U.S. Open; we said we were coming to see Roger at the U.S. Open. . . . And then everyone, they texted me, and said, ‘Fabiano, you’re not seeing Roger.’ ‘Aaarrrrggghhh, I know. You don’t have to tell me that!’ ”

Still, they traveled. They fell in love with the event, even if the Roger-lessness makes it “a little less beautiful.” Besides, if they can’t root for Federer, they can root for 17, which they do. That’s Federer’s number of Grand Slam titles, and they don’t want anybody eclipsing that.

That means rooting for Andy Murray, who at 29 has won three Grand Slam titles, thus does not figure to howl at 17.

As they all keep hope for more Roger in future years, a U.S. Open without Federer has given them a glimpse of the great, blank, post-Roger beyond. They make frequent use of the word “grace,” in which the Lawtons of Sag Harbor liken Federer to Derek Jeter. “There’s supreme authority in his calm, and I admire that,” Glenn Lawton said of Federer. Seated with Humphreys at the food court, the Brooklynite Bruce Schaffer noted Federer’s 11 sportsmanship awards in the last 12 years, as voted by peers, and said, “I like knowing there are people in this world who in addition to being competitive are also, generally speaking, gentlemen.”

Fabiano Menke, all the way from Brazil and thinking of returning next year, may have formulated one of the all-time descriptions of the 35-year-old man from Switzerland and, really, anymore, Earth.

Amid a long paragraph about Federer’s attributes, Menke said, rather wistfully at this U.S. Open, “He doesn’t sweat like the others.”