Roger Federer has been through greatest-of-all-time discussions plenty of times, long before Novak Djokovic went on his yearlong tear in 2015 that ranked among the best that any male tennis player -- including Federer -- has ever had.
As a younger man, Federer successfully chased down Pete Sampras before enduring Rafael Nadal's determined attempt to eclipse him.
Now, the 34-year-old Swiss star finds himself at the center of another debate.
The most provocative question in tennis heading into the new year and the Jan. 18-31 Australian Open is whether Federer's status as best ever needs to be reappraised, given how rapidly Djokovic is gaining on him in the chase for most Grand Slam titles.
"I don't buy it -- Novak still has a long way to go," says former U.S. Davis Cup captain and current ESPN analyst Patrick McEnroe.
"If Djoker repeats another year in 2016 like he had in 2015, every Slam title he wins puts him more in the equation," counters Brad Gilbert, fellow ESPN commentator and former coach of Andre Agassi, Andy Roddick and Andy Murray.
It's the same conversation Federer found himself tossed into with Nadal during those years when the Spaniard was pulling away to a 23-11 advantage in their head-to-head matches and laying his own claim by finally winning at Federer's beloved Wimbledon in their classic 2008 final. But since getting to 14 Slam titles, three fewer than Federer's record of 17, the 29-year-old Nadal's dip in form because of injuries and occasional spasms of self-doubt is also a cautionary tale about anointing Djokovic too soon.
Djokovic now has 10 majors. Only the French Open title has eluded him. At 28, the Serbian-born star is six years younger than Federer, and his major-title collection is accelerating just as Federer is understandably slowing down.
But is it safe to presume the wondrous results Djokovic has had are something he can maintain? Can he possibly duplicate last season, when he finished 82-6, made the finals in 15 of 16 tournaments and won three of the four Grand Slam titles? Can he keep improving, stay healthy and resist letting self-satisfaction or ennui creep in as he chases his fifth year-end No. 1 ranking, matching Federer's career total?
Tennis analyst Mary Carillo says she and former world No. 1 Ivan Lendl were kicking around those very topics when she interviewed Lendl at a recent exhibition tournament.
"I asked him who among the Big Three [Federer, Nadal or Djokovic] would end up with the most majors," Carillo says. "With his typical and inimitable, mathematical Czech mind, Lendl immediately logically crunched the numbers. ... [He] points out the cruelty of the math: For every major you don't win, someone's tally is going up. ...
"Novak is younger, fresher and the best player, arguably, on every surface. If he can maintain his current output -- a few majors a year, at least, for the next three or four years -- then he can surpass Federer and everyone else. I think Lendl is exactly right about this."
And if Lendl is?
"I'd have to give Novak best-ever status," Carillo says. "Especially because of the era he competed in."
If Djokovic does get off to a fast start and wins the Australian for the sixth time, we'll also have to see whether the talk of him pulling off a calendar-year Grand Slam sweep wears on him the way it did Serena Williams last year.
"I understand that those speculations are in place because of the season I had in 2015," Djokovic said two weeks ago when he kicked off his season in Qatar. "I can't really predict what's going to happen or guarantee if I'm going to win any of the Grand Slams or all four Grand Slams. Of course I'm going to try."
No all-time great is judged solely by the numbers. They're also defined by the aesthetics of their game and the anecdotal evidence. How do their skills compare to other greats? How about their heart? How did they fare in their epic matches? Did they rise up in the biggest moments -- or not? How many consecutive years did they win, dominate and fight?
Djokovic certainly has a lot of factors flowing in his favor.
Nick Bollettieri, who has reared a lot of tennis champions in his time, has repeatedly called Djokovic "the most complete player ever."
Nadal is the more powerful grinder and best clay-courter ever. Djokovic is generally considered the best service returner of all time and, much like Federer at his best, has improved to the point that he can switch from defense to offense in a blink. Federer has a terrific forehand, a sneaky-good serve. He and Djokovic move like gazelles and excel at thinking their way to wins even when their shot-making is off. All three men have demonstrated the fitness to outlast rivals in a match.
But Gilbert says something else distinguishes Djokovic to him.
"If you look deeper," Gilbert says, "to me the most interesting thing about Djoker is, Rafa was dominant at a young age, Fed was racking up a lot at young age, but Djoker, he didn't really start doing that until 2011. He'd been [ranked] behind those other two guys for the longest time, just chasing. And I'm not sure I've ever seen a great player chase longer to get his art than him. He chased for such a long time."
ESPN tennis analyst Cliff Drysdale, who played tennis in the same era as Rod Laver, says reopening the debate about Federer and Djokovic's place in history is a naturally recurring part of sports when greats come along and elevate the game.
"In my own mind, I felt like, when Jimmy Connors was at his best, I thought he was the best player who ever played," Drysdale says. "Then along came Borg and Ivan Lendl, and I wondered how in the world anyone could play tennis better than that? Then there was Pete Sampras. ... It does just keep getting better.
"I'd still have to go with the great Roger Federer [as the best ever]. But to me, this is a golden era."
Djokovic was asked at the end of last season whether he thinks about overtaking Nadal and Federer for Slam titles. He said, "Of course I care about it ... [But] I don't have a sense of urgency. I don't chase anything. I just try to be in the moment and see where it takes me."
There's a wing of the tennis world that argues Federer is actually muddying or even damaging his legacy by playing so long. The thinking is that Federer, who hasn't been the year-end No. 1 since 2009 or won a major since Wimbledon in 2012, should have literally quit while he seemed untouchable.
"Well, I don't agree with that," McEnroe says.
"Lord, no," Carillo agrees. "He can still play tennis that no one else plays."
"I think the argument is nonsensical," Drysdale adds. "Roger is always in the running. He's always still playing on the final weekend. He's never not a factor."
At minimum, Djokovic has revived the best-ever debate. And if he keeps winning majors, it will help him that, unlike Federer's losing record against Nadal, his head-to-head records against both men bolster his case.
Djokovic is a 24-23 against Nadal and 22-22 versus Federer.
But be careful about writing off Federer too soon.
Djokovic did beat him in their 2015 Wimbledon and US Open finals. But it was Federer, the "old" man, who handed Djokovic three of the six losses he had in all of 2015. No one else beat Djokovic more than once.
So it won't be a shock if Federer finds a way to make that "cruel math" that Lendl invoked work for him, starting at the Australian Open. He's fully capable of hitting the pause button on Djokovic's pursuit of him.
Something Federer noted in London the day before knocking off Djokovic in round-robin play at the year-ending ATP World Tour Finals in November applies to all of 2016.
"I don't think I'm that far off," Federer said. "I'm curious to find out."