Say the NBA tried an experiment. Instead of playing every game on a standard hardwood court, the league introduced four new surfaces — materials that not only looked different, but also fostered wildly divergent styles of basketball. In the northern states, the new rye-grass lawn courts made dribbling far more difficult; the ball tended to bounce low, skid, and jump in unpredictable directions, forcing coaches to develop new strategies based on stationary passing and movement off the ball. In the South, courts made of crushed brick presented exactly the opposite problem — the floor now had so much bounce that players had to raise the height of their dribble, taking driving out of the game and leading to a cautious, slow-paced style that emphasized avoiding turnovers over everything else. On the coasts, the courts were still made of maple, but the wood was sourced and treated differently; East Coast teams practiced more or less the form of basketball we’re used to today, while the game on the West Coast slowed down almost as much as on the red courts of the South.

Now say that instead of happening overnight, this situation evolved over a full decade, as individual arenas tinkered with their product and new conventions took hold. In time, players whose talents suited grass-court basketball filtered onto the Pistons and the Bucks. Players with natural clay-court games went to the Pelicans and the Heat. In the twilight of his career, LeBron finally made it to the Knicks. Franchises transformed to specialize in their own particular surface; on floors they weren’t adapted to, they faced huge disadvantages. Champions had to be versatile, but no one could expect to succeed on more than two, maybe three surfaces. The game fragmented to the point that people started talking about splitting up the season and staging four separate championships.

So if that all happened … it would be huge news, right? Easily the biggest change in basketball since the advent of the shot clock, and probably the biggest change ever. When you talked about the NBA, this would be the first thing that came up. If no team was able to dominate the regular season, you wouldn’t just say “Wow, it’s an era of parity,” like the reason was some huge mystery — you’d talk about how the different surfaces made it that much harder to field a historically elite team. Studio crews would be doing goofball features bouncing beach balls on the different surfaces and speculating about how the ’90s Bulls would do in the post-wood era. Charles Barkley would be Photoshopped into, and might at some point wear, a grass suit. The transformation of the game would be too glaring to ignore.

So why aren’t we having this conversation about tennis? Over the past 10 years, something resembling this story has played out across the Grand Slam tournaments, only exactly in reverse: What used to be four radically different surfaces, requiring four radically different styles of play, have become increasingly homogenous. This is a major factor — arguably the major factor — in the current state of the game, particularly in men’s tennis, where it has helped shape both the Nadal/Federer/Djokovic/Murray golden age and the slow-paced, relentless, defensive tennis that more and more seems to define it. There’s a serious argument to be made that Rafa Nadal, Novak Djokovic, and even Roger Federer could never have won so many majors if they’d played in an era before the biggest competitions started rewarding similar skill sets. But instead of analyzing this to death the way we would with the NBA, the tennis world treats it as a sort of open secret. No one talks about it. (Or rather, only some people talk about it; more on that in a minute.) Instead of putting on a grass suit, John McEnroe just rhapsodizes about the talent and heart of the top four and leaves surface technology pretty much out of it.

That’s not to throw stones at McEnroe, by the way — I’m guilty of this, too. When I’ve written about tennis, I’ve spent more time comparing Nadal to Hector from the Iliad than breaking down the soil composition outside Troy. I realized this needed to change after Nadal’s recent win in the French Open final, when I wrote a column arguing that if Rafa could win more majors than Federer while playing in the same era, he should be considered the greatest player of all time. My friend Toby, who’s the most obsessive tennis fan I know (and who cultivates a luxuriant bitterness toward Nadal, if that’s relevant), started sending me videos of fast-court tennis from the ’90s and asking if I thought Rafa could win majors under similar conditions.

At first I blew this off. Like most tennis fans, I was aware that the courts had slowed down. But I was happy to treat that as a small thing compared to modern-day improvements in strength, fitness, strategy, and vision among the top players — your basic generational-sports-vanity cocktail. But the more I replayed the videos, the clearer it became that Toby was right: Nadal would have been slaughtered like a wedgie-afflicted calf at any Wimbledon of the high Sampras era. Which doesn’t mean I was wrong to say that Rafa’s got greatest-of-all-time potential; after all, Pete Sampras might not have won seven Wimbledons in Nadal’s slow-grass era. But isn’t it a wrinkle that at least belongs in the conversation?

Let’s back up a little. It’s not hard to see why tournament organizers started flirting with lower court speeds. It’s for the same reason Federer seemed like such an electric presence when he came onto the scene in the early 2000s: Because the power game that emerged in the ’90s in the wake of Ivan Lendl and composite-racket technology was widely perceived as boring. Forum oldheads now tend to talk about ultra-fast, three-shot-rally tennis as though it’s a pure form of the game. But at the time, matches like the 1995 Wimbledon semi between Sampras and Goran Ivanisevic (which featured 59 aces and a second set in which Ivanisevic not only didn’t drop a single point on his serve, but won 17 of his 20 service points with one shot) were seen as crude and dull compared to the more tactical, angle-savvy tennis of the McEnroe-Borg era. All sorts of proposals were floated to extend rally lengths: larger balls, brighter balls, restrictions on graphite; McEnroe and Martina Navratilova even advocated switching back to wooden rackets. What almost certainly ended up happening was that individual tournaments started fiddling with ball choices, slowing down faster courts, and making all court types more similar to each other.

Now, keep in mind that there’s no proof of a lot of this, because the powers that be are understandably loath to come out and say “Hey, so we’re manipulating the raw materials of the game to make it artificially more entertaining.” But there’s some stuff we do know. In 2001, for instance, Wimbledon switched from a 70 percent rye grass/30 percent creeping red fescue mix to an all-rye formula supposedly designed to improve the grass courts’ hardiness. The All England Club denies that the switch was made to slow down points, but that’s clearly been the effect, and it’s become mysteriously more pronounced over the years. Here’s a BBC video that illustrates the post-bounce trajectories followed by two very similar Federer serves, one from 2003 and one from 2008, and while the difference could be attributed to weather, or humidity, or a butterfly flapping its wings in Kyoto, it’s consistent with the way players have described their experience over the last 10 years:

The Australian Open — traditionally the slower of the two hardcourt majors — switched from a surface called “Rebound Ace” to one called “Plexicushion” in 2008. There’s disagreement over whether Plexicushion is a higher-bouncing surface than Rebound Ace, but because it’s relatively rough, it has the effect of “fluffing up” balls, causing them to fly at slower speeds and increasing the odds of, I don’t know, six-hour baseline slugfests that leave both players unable to stand. The U.S. Open — the faster hardcourt major — hasn’t officially changed surfaces, but organizers added more sand to its acrylic-paint mix between 2001 and 2003 to slow down play. And as with Wimbledon, the trend has seemingly progressed since the surface change without anyone exactly admitting it’s happening. After a particularly slow tournament in 2011, there was widespread unanswered suspicion that the formula had been tweaked again.

The slowing of the courts had the intended effect of lengthening points, albeit with the possibly unintended consequence of making the three faster majors look more like the French Open. It also had another effect that, arguably, turned out to be a much bigger deal. It helped make it possible for the same players to dominate every Grand Slam tournament. To what extent that would have happened anyway — to what extent it’s a question of pure talent and training vs. the manipulable parameters of the sport — is an open question, and always will be. But if you lowered the basketball rim to 8 feet, or raised it to 11, you would instantaneously alter the composition of the next generation of great basketball players. By the same token, there’s no doubt that staging all the major tennis tournaments on relatively similar surfaces confers a huge advantage on players who happen to excel on that type of court. When Bjorn Borg went from winning the French Open to winning Wimbledon a few weeks later in 1978, ’79, and ’80, it required a tremendous leap of versatility; when Nadal did it in 2008 and 2010, or when Federer did it in 2009, the leap it required was much smaller.

Maybe relevant here: The same four players have won every major and Olympic gold medal except one in the past eight years. That’s 34 big titles for Federer, Nadal, Djokovic, and Andy Murray, and one for every other tennis player on earth.

Also relevant: That’s totally unprecedented in the history of tennis going back to the 19th century.

It’s easy to see how this state of affairs has helped Nadal and Djokovic, defense-minded baseline warriors who — and think about this — have essentially become the first great tennis players in history to play all their majors on surfaces that suit their games. On a slower court, Nadal has more time to run around his backhand, more time to line up shots, more time to exploit his defensive strengths, and more chances to deploy his personal topspin arsenal. Add all that up and obviously it would tip things in his favor.

The more interesting question for me is how it has affected Federer. Roger tends to complain about slow surfaces and slow play (see, for instance, here, here, here, here, here, and here), and in theory his attacking game would benefit from faster court speeds. On the other hand, he won 13 of his 17 majors in that 2005-13 span, won all seven of his Wimbledons after the switch to 100 percent rye grass, won all five of his U.S. Opens after the court was slowed down, won an Australian Open on Plexicushion, and generally hasn’t been too hampered by the Great Surface-Speed Convergence of Aught-Whatever. So have we misread Federer in some mysterious way — is his cool, thoughtful game actually helped by slightly slower play? Or is he such a phenomenal talent that he won 17 majors while the organizers of his own sport were essentially working to help his biggest rival? If conditions had stayed as they were in the Sampras era, would he have won 30 majors, or four, or none?

Well, welcome to the “guess what, it’s impossible to compare eras” section of today’s column. Conditions in sports change continuously, and athletes can only play the game as it exists when they play it. Asking how Federer or Nadal would have fared in the ’90s is not much more sensible than asking how Bill Tilden would have fared in shorts, or which 5-foot-9 shooting guard would be the best-ever on a grass basketball court with an 8-foot hoop. I’m increasingly certain that the question of who’s the greatest player ever, in any sport, belongs more to folklore than to science — that however you quantify or compare, the king of all eras is whoever’s story inspires the most excited acclamation. And yes, that means the answer people accept will always drift toward the present, because people are always most excited about whomever they saw play. This is only sports, and while I’m all for letting the stats influence the legends, in the end the legends have lives of their own.

If you’re a super hard-core tennis fan, then you’re probably still annoyed by something I wrote toward the beginning of the piece, when I said that the convergence of court speeds is something no one talks about. That’s totally untrue; people talk about it all the time, to the point that, among the sorts of tennis fanatics who hang out in forums, or know in what month the PBZ Zagreb Indoor is held, it’s practically a cliché. Players talk about it constantly, columnists write about it, it inspires backlashes and counter-backlashes. It’s a known thing. But here’s something else that’s a known thing: Every conversation in tennis takes place on two levels. There’s the level of insiders — players, reporters, and full-time fans, people who follow the tour year-round and know the rankings down into the 50s. And then there’s the level of fans who mostly wake up to the sport during Grand Slam tournaments. This second group of fans may still be deeply serious about the game — watching hundreds of hours, getting up at odd times to take in early-round matches, being glued to the sport eight weeks a year — but they’re not really tuned in to the day-to-day insider conversation.

This second set of fans, it almost goes without saying, is larger than the first and, in the long term, more influential in defining players’ legacies. And it would be really easy to belong to the second set and have no idea that the speed-convergence debate even existed. It’s occasionally mentioned on TV, but not nearly as insistently as the simple heads-up major count. It shows up in print, but not nearly as often as some Homer-alluding goon telling you that the G.O.A.T. race is all Nadal vs. Federer. You have to read below the surface (ha) of the coverage to get the extent of the idea. Tennis, unlike the NBA, just has ways of airing its issues without ever alerting even fairly serious fans.

And that’s too bad, because it’s with the second group of fans that the folklore of the game ultimately has to resonate. We should be discussing this more during majors and more on TV. John McEnroe should put on that grass suit. Not so we can stage some faux-science Discovery Channel inquest into how Andy Murray would look on peat moss, but so that fans can fold it into their ongoing sense of Nadal’s story (the second-best player in the world, who persevered through injury to surpass the best, though maybe with some help from tournament organizers), or Federer’s story (so good he won more majors than anyone during a time when the major tournaments were arguably designed to punish his natural skills), or Sampras’s story (won 14 majors at a time when the Grand Slam tournaments played almost like four different sports), or Djokovic’s, Borg’s, Roy Emerson’s, or Rod Laver’s stories, and decide for themselves which one they find most thrilling.