Professional sports, by their nature, are in constant transition. With players now peaking in their mid- to late 20s, tennis experiences a considerable annual turnover.
"It's like that moving walkway at the airport," said Brad Gilbert earlier this week from Winston-Salem, North Carolina, where he called matches for ESPN. "When you get off, it keeps on moving. Those tournaments keep on coming."
Roger Federer and Rafael Nadal won at least one Grand Slam title between them for a dozen straight years, from 2003-14. Although that streak ended last year, 2016 looks and feels like something approaching a sea change on the men's side.
New York, in particular, will offer a distinctly different vibe. For the first time this century, Federer will not be present at the US Open, which begins Monday. The five-time champion will presumably be back home in Switzerland, surrounded by his wife and four children, rehabbing knee and back injuries with an eye on next year.
"I'm trying to focus on getting back, being healthy and remaining motivated," Federer told reporters earlier this week while in New York for a promotion. "It's not a struggle to go to the gym or the physio room and do my exercises."
More specifically, Federer targeted early January to make his comeback, at the Hopeman Cup and then the Australian Open.
"I remain very upbeat but in a way it's painful to be here, and it was hard watching the Olympics and not playing," he said.
Gilbert, who coached Andre Agassi back in the day and later, Andy Roddick, Andy Murray and Kei Nishikori, turned professional in 1982 and played through 1995. In 35 years, he's seen many generations of athletes.
"It will feel different," Gilbert acknowledged. "We didn't have Fed in Canada or Cincinnati, but for the casual fan, the void will be more noticeable at the US Open. In a way, his absence will help make them appreciate what he's done for the game."
The 35-year-old Federer made the difficult decision to shut it down after Wimbledon. He still ranks No. 3 among ATP World Tour players, but 28 matches are his fewest in a season going back to his first, in 1998. For the first time since 2000, he will go an entire year without winning at least one title.
"It's going to be a different energy," said Justin Gimelstob, who will be in New York as a Tennis Channel analyst. "We'll miss his aura and his presence. But the US Open is still the US Open. There are a lot of intriguing storylines out there."
Nadal, now 30, won nine of 10 titles at Roland Garros but had his worst result in Paris in May after granting a third-round walkover victory to fellow Spaniard Marcel Granollers. That left wrist injury also caused Rafa to miss Wimbledon before he reached the bronze-medal match at the Rio Olympics. For Nadal, this will be only the second appearance in New York in the past four years.
Even Novak Djokovic, the best player in tennis over the past five-plus years, has seen his grip on the game become a tad looser. The flexible, fluent Serb won the season's first two majors, in Melbourne and Paris, then watched as Murray won Wimbledon and the Rio Olympics. But Murray wasn't the one who beat Djokovic in either of those events.
After losing in the first round at Rio to Juan Martin del Potro, Djokovic -- citing a "recurring left wrist injury" -- elected to skip Cincinnati, while Murray reached the final Sunday, only to fall to Marin Cilic.
The condition of Djokovic's wrist has gripped the rabid slice of tennis fans on social media. New York should provide some answers.
Yes, after enormous anticipation and advance publicity, we're into a new era. The 30-somethings are giving way to teenagers and players already formidable in their early 20s.
Federer, of course, was once a teenage phenomenon.
He was 16 years old when he played his first ATP-level match, in Gstaad, Switzerland, where he lost to Lucas Arnold Ker. The second match didn't go any better; Federer lost in Geneva to Orlin Stanoytchev. In his third tournament, in Toulouse, France, he got to the quarterfinals and this pushed his year-end ranking all the way up to ... No. 302. A year later, he was No. 64.
In his first US Open, in 2000, Federer advanced to the third round, where he dropped a fourth-set match to future two-time French Open champion Juan Carlos Ferrero. Four years later, Federer won his first US Open title -- his third major triumph of the year.
In recent years, however, men's tennis -- at least at the top -- has been a remarkably stable enterprise. Since Marat Safin won the 2005 Australian Open, Federer, Nadal, Djokovic and Murray have, astonishingly, won 42 of the past 46 Grand Slams.
In that time, no fewer than 17 women -- including Flavia Pennetta, Marion Bartoli and Samantha Stosur -- have won majors.
Federer hasn't won a major since Wimbledon in 2012, but going forward, he still sees himself as a major player.
"I took this decision [to have surgery and take time off] because I see myself still playing for numerous years," Federer said. "Otherwise, I would have just said, 'Who cares. I'll just somehow play in some shape or form, unhealthy, healthy, it doesn't matter at this point. I'll just fizzle out at the end of my career.'
"But I care too much about the game and my health that I still believe something is possible for me down the road in tennis."