Shortly after Novak Djokovic mastered Rafael Nadal at this year's Australian Open final to lock down his 15th Grand Slam singles title, the simmering GOAT debate came to a boil again. We immediately asked whether we need to start thinking about the top-ranked Serbian as the best ever. But it raised another interesting question: Is it really possible the three greatest players of all time are active right now?

Sure, it's feasible, but it's highly unlikely. This embarrassment of generational riches -- Roger Federer owns 20 major singles titles, Nadal has 17, just two more than Djokovic -- suggests our metrics for determining the GOAT are far too simplistic. The cumulative success of this trio is so overwhelming that it makes you wonder if it's even fair to compare them to their predecessors. If you acknowledge that, the GOAT debate changes drastically.

Roger Federer has more major titles than anyone, but he's playing in a much different era from the stars of yore. Chaz Niell/Icon Sportswire

A little history is in order, because it helps us understand how we got here. Up until Pete Sampras hit his stride, most people assumed Roy Emerson's record of 12 major singles titles would not be broken in the Open era. The game had changed too much, they thought. The depth in the ATP was too great. Nobody was even bothering to play the Australian Open, where Emerson had collected six of his singles titles.

But two critical things were also happening: the rehabilitation of the Australian Open (a tournament that icons Bjorn Borg, Jimmy Connors and John McEnroe had played a combined total of just eight times in a total of 49 years of Grand Slam play) and the homogenization of the Grand Slam surfaces.

By the time Sampras emerged, it was no longer a three-Slam game. The opportunity to win a major every year increased by 25 percent. Just as important, over time, the hard courts at the US Open and Australia became medium-to-fast-paced (depending on the year), the fast grass at Wimbledon was slowed to hard-court-like speed, and the changes in equipment and court maintenance helped eliminate the dreaded clay-court specialists who once terrorized seeds at the French Open.

"One of the ingredients in the success of the Big Four, other than their being incredibly gifted, is that they don't have to adjust as much to different styles or surfaces," Tennis Channel analyst Paul Annacone, who has coached both Sampras and Federer, recently told ESPN.com. "Back in the day, the discrepancy between surfaces opened opportunities and presented challenges. Now the only thing different is the movement, but even that's changing. It used to be you'd slide on clay; now guys are also sliding on hard, so how different can the movement really be?"

True, Nadal is still a different player on clay; his exceptional record (86-2, 11 singles titles), which is unmatched at any major by any player, proves it. But apart from that detail, today's majors don't reward some styles while punishing others. They favor the most talented all-court players and reduce risk for them. Hence, the rich get richer.

The speed at which these three titans eclipsed Sampras' record, after Emerson's had stood for so long, means a basic haul of major titles is just the entry fee into the GOAT debate. Other aspects of the record then become much more important. Those include head-to-head records, success in other first-rate events (like Masters 1000s) and overall winning percentage.

Let's not forget that because he spanned the amateur and Open eras, Rod Laver was denied the opportunity to play majors for six years starting at age 24, the year after he completed a calendar-year Grand Slam. Laver returned in the Open era and then accomplished another sweep of the majors in 1969. He remains the only Open era player with two calendar Grand Slams.

All this suggests there's no real GOAT -- maybe there's a herd of them.