It's a staggering number, 88, and by the end of this potentially historic year in tennis, it could grow considerably larger. It takes a long time to accumulate 88 Grand Slam titles. And it certainly takes a toll. That's why it's a good idea to enjoy the spectacle created by these players. Because you have to wonder -- how much longer can it last?
But what a spectacle it promises to be in 2016. Serena is almost sure to at least tie, if not surpass, Steffi Graf as the Open era's singles leader with 22 majors, one more than the current world No. 1. But Serena is 34 years old.
And so is Federer, who's still Novak Djokovic's strongest challenger -- and hoping to plug the only significant hole in his résumé with a singles gold medal at the Rio de Janeiro Olympics.
The Bryans, who will turn 38 in April, are Grand Slam hungry after having been shut out in 2015. And while Venus will be 36 before the tennis year is out, she's being propelled by a 2015 resurgence that landed her back in the top 10. Nadal is the baby in the group. He'll turn 30 before the next French Open final, but he's injury-prone and battle-scarred, seemingly a very old 29 whose future is clouded.
Sure, tennis has become a veteran-friendly sport. That's crystal clear. But the reality is these icons all could break down or tire out in 2016. It makes you wonder: What happens next?
"It would be kind of like when somebody dies," said ESPN commentator Chris Evert told ESPN.com, pondering a tennis landscape robbed of Federer. "I know, because I had that feeling when Bjorn Borg suddenly retired [in 1981]. He was so much bigger than the game, it seemed.
"I was just like: 'Oh my God! What now?' I thought tennis would crash. Prize money would go down. Nobody would watch. But six months later, he was replaced. Life goes on. The game goes on. It gets bigger and better."
How much better? Perhaps Novak Djokovic better.
That's the ace-in-the hole for the men's tour, as Djokovic's growing portfolio attests. With 10 majors and a seemingly insatiable appetite for the obligations as well as the rewards of a living legend's life, Djokovic seems poised to tape over the Swoosh and step right into the tennis shoes Federer leaves vacant. In fact, perhaps Federer is hanging on partly to deny Djokovic that satisfaction.
Djokovic is aware that the main reason the public has yet to fully embrace him is Federer. The world at large simply loves the man. The top-ranked 28-year-old has handled this beautifully, though, with nary a complaint while building his own reputation.
"Djokovic has always been an extremely thoughtful young man," ESPN commentator Cliff Drysdale told ESPN.com. "He is every bit as ambassadorial as Federer."
Ambassadorial, but with an edge.
"Djokovic has that no-BS charisma on the court," hall-of-fame coach Nick Bollettieri told ESPN.com. "Where Roger is like Mr. Clean, Djokovic gets down and dirty. People like that, too."
And Nadal? He needs to prove that he's been in a slump rather than burned out as a Grand Slam champ.
"The pattern suggests that Rafa comes back really strong from an off year," ESPN's Pam Shriver told ESPN.com. "But those off years were all due to injuries. Last year, his problem was mental frailty, so this is really new territory for him, and us."
But even if the stars of Federer and Nadal dim, or are extinguished, Andy Murray and two-time Grand Slam champ Stan Wawrinka appear to be in it for the long haul. Murray is just 28, and while Wawrinka is 30, he's just recently found his Grand Slam identity. His spirit is fresh.
Murray has a serious Djokovic problem, having lost 10 of their last 11 matches. Oddly enough, Murray fared better before both men rounded out their games.
"Everything to be done for Andy has been done," Bollettieri said. "He's moved closer to the baseline; he's improved his serve; he's developed a hell of a running forehand. He just suffers from a matchup problem because Novak does similar things, but does them all a little better."
Unlike Murray, Wawrinka can blast Djokovic -- or anyone else -- off the court. Throw a few other dangerous players into the mix, guys such as Marin Cilic, Jo-Wilfried Tsonga, Tomas Berdych and Milos Raonic, and Evert's words that tennis will prevail begin to sound prophetic, even if you agree with Shriver who said: "When Roger retires, you can't just have him go off to raise four kids. He'd want to do that, but tennis needs him in some capacity. He's a beloved global figure."
The future of a women's game without the Williams sisters is opaque simply because, while there are women ranked No. 2 and 3 and so forth, there appears to be no true successor comparable to a Djokovic, never mind a fleet to match the likes of the Serb, Murray and Wawrinka.
Maria Sharapova, presently No. 4, is the heiress apparent. But as Shriver said, referring to Sharapova's injury history, "She's just 28, but her arm is pretty old."
Evert, whose forte as a player was the mental game, has criticized this as a weak era, a period during which, she said, "Nobody is really stepping up."
When the Williamses move on, though, it will theoretically motivate youngsters (currently, think Garbine Muguruza, Belinda Bencic or Eugenie Bouchard). It also ought to rekindle the determination in oppressed but still youngish veterans such as Angelique Kerber, Agnieszka Radwanska and Simona Halep. Yet without a dominant, familiar star, the WTA could find itself in big trouble.
"The Williams name will be hard to replace," Butch Buchholz, the pioneering former player who created and ran the combined Miami Masters for many years, told ESPN.com. "Maria Sharapova will sell tickets, but the tour has a lot of players with great games but unfamiliar names. They will have a marketing and PR job to do, and they should do it before Serena and Venus are gone."
If anyone in the game seems irreplaceable these days, though, it's Bob and Mike Bryan. Not only are they the all-time Grand Slam champion doubles team (16 titles), they played a critical role in preventing a struggling tour from all but eliminating doubles from the tour. No player(s) have ever worked as hard to promote the game.
You have to wonder, what happens to doubles when the ATP no longer has a successful, easily marketable team with a strong, simple identity -- and a willingness to walk the extra mile to promote the game. One enormous problem: Teams form and break up too quickly, irrespective of factors like shared nationality.
Thankfully, tennis may have some time to ponder this challenge. As Wayne Bryan, the twins' father and career mentor said in an interview with ESPN.com, "Guys like Daniel Nestor, Julian Knowle and Leander Paes are all in their 40s and still competing at a high level.
"[Bob and Mike] haven't mentioned retirement to me yet, not once. If anything takes them off the tour it probably will be kids and the demands of family life."
Here's an idea: Federer has two sets of twins. He could begin grooming them now for the eventual task of making the world forget the Bryan brothers. But it appears that Federer still has his own work to do on the court.
"I can't imagine that Djokovic won't have a slip here or there before Federer and Nadal are done, which is why I'm confident that both those men will win at least one more major each," Drysdale said.
Then, reaching for a little help from Broadway, he added: "Anyone who doesn't believe me, I just tell them, 'Just you wait, Henry Higgins, just you wait.'"