What are Rafael Nadal's chances to surpass Roger Federer for the all-time lead in Grand Slam titles? Only 21.5 percent, according to 538.com.

No, really. They did a whole chart, mapping out possible scenarios and their likelihoods, and that was the prognostication handed down by the well-known forecasting outfit, albeit before Nadal beat Novak Djokovic on Sunday to win his 14th major trophy, just three shy of Federer's mark.

The conversation around who is the greatest of all-time is more complicated than Grand Slam trophies, but the current era of players has grown up entirely in an age when those four tournaments were the Holy Grail of the sport. It is therefore reasonable to revere that number as the benchmark when comparing Nadal to Federer to Djokovic to anyone else who may come along.

But even that has its nuance. Another chart in that same article at 538 shows how the curve for age of a men's singles Grand Slam winner peaks at 24-25. Ages 22 and 26 are the next most common for winners. Nadal is 28, where the real doldrums of the curve downswing begin.

We point this out because Federer is 32. This means that when the Swiss was in that 22-26 sweet spot, Nadal was just 18-22 himself. Djokovic was 17-21. Same for Andy Murray, the only other multiple Slam winner of this generation so far. Federer won the 2008 US Open shortly after his 27th birthday for his 13th Grand Slam title. He has just four in the years since, when those other three players have swung through their primes.

Nadal not only had to live in Federer's shadow early on, but he's had to contend with Djokovic and Murray -- both just a year younger than him -- for the majority of their primes. Nadal caught the late end of Federer's best years and pretty much all of Djokovic and Murray's. Nadal is 23-10 against Federer all-time, 9-2 in majors. He's 23-19 against Djokovic, 9-3 in majors. Against Murray? 15-5 and 7-2 at majors.

Given all that, you could well argue that Nadal has already earned the title of greatest of this generation, if not ever. But if you still need some convincing, here are the five questions that will determine whether Nadal overtakes Federer in the one stat everyone holds most dear:

Is Federer really done winning majors?

Wimbledon will mark two years since Federer last hoisted a major trophy. After a record run of 36 straight majors from 2004-2013 in which he made at least the quarterfinals, he has now failed to get that far three times in the past four Slams. Obviously, if he can find a way to scratch out another major title or two, he makes the road to passing him immensely more difficult for Nadal.

How much does Nadal's body have left?

Nadal's injury issues have been well documented throughout his career, and we saw a lingering back issue crop up briefly Sunday. He has missed four majors altogether since 2006 and has had ailments affect his chances at winning others. With the pounding he puts on his body every match and his age creeping toward 30, it is a real question whether he can stay healthy long enough to win four more of these titles.

How long can Nadal keep that stranglehold on the French Open?

Though a bit outrageous to think, it's possible -- if he can stay healthy -- that Nadal could pass Federer on the strength of French Opens alone. His dominance over the years is enough to make a case for really saving himself for four more runs at Roland Garros. Almost assuredly, he'll need at least two more titles in Paris to have a shot at the record. And with Djokovic shrinking the gap between them on clay, that may be easier said than done.

What can Nadal do at the other majors?

This all, of course, becomes much easier if Nadal can tick off a couple of titles at the other Slam venues. But Wimbledon has been a house of horrors for him in the past couple of years. He has lost twice in the Australian Open final in the past two years and missed it altogether in 2013. His US Open title last year is his only non-French major title since 2010. The next three Slams off the surface he loves most are critical toward getting to -- and passing -- 17.

How good is that next generation?

Murray may still have room to grow. Ernests Gulbis announced his intentions with a strong semifinal run in Paris. The young Canadian Milos Raonic is making waves. Others like Kei Nishikori and Grigor Dimitrov have given Nadal fits in big matches recently and still appear to be on the rise. It's enough to contend with Djokovic every tournament without having a new crop of hungry youngsters nipping at your heels.

The argument over greatest of all time can go on forever, and that's what makes it fun, but Nadal's accomplishments in one of tennis' most talent-heavy eras are truly remarkable.