Empires fall, but seldom all at once. They fall slowly, piece by piece. Barbarians mass on the borders. Unrest builds in the capital. Weak rulers poison strong ones. The army wins battles but loses provinces. The collapse can take years. Afterward, no one is able to pinpoint the precise moment when the old order failed. There was no single moment. There was only an epoch of decline, followed by the spreading realization that what used to be the empire has now become something else.

Thomas Cole, New-York Historical Society

Between July 2003, when Roger Federer won his first major title, and September 2013, when Rafa Nadal won his 13th, men’s tennis contested 42 Grand Slam tournaments. Thirty-eight of them were won by the same four players. If you’re a tennis fan, you’ve seen these numbers parsed a thousand different ways; still, they remain astounding. Federer and Nadal alone combined for 30 of the 42. Novak Djokovic took six, Andy Murray two. Of the four majors to slip through the Big Four’s grasp during that span, three came early in Federer’s prime, before Nadal had played a single clay-court tournament in Paris: Andy Roddick’s U.S. Open in 2003, Gaston Gaudio’s French in 2004, and Marat Safin’s Australian in 2005. From May 2005 to the end of 2013 — a period of more than eight years — the only non–Big Four major winner was Juan Martin del Potro, who took the 2009 U.S. Open and threatened to crash through into the game’s top rank before his own body turned against him.

Whatever the reason for the consolidation of power — changing racket technology, the convergence of court speeds, modern fitness regimens, a pure freak of genetics — the decade of big-tournament dominance by a single quartet of players was unprecedented in the history of men’s tennis. The 1970s and early ’80s are remembered as an era of loud, colorful talents, but McEnroe and Borg and Connors didn’t win everything; there was room for players like Guillermo Vilas — who won four majors between 1977 and 1979 without ever being ranked no. 1 — or Brian Teacher or Johan Kriek, to lift occasional trophies as well. Pete Sampras and Andre Agassi towered over the 1990s, but even during their peak years — say, 1993 to 2001 — they won only around half of all majors. Federer and Nadal, by contrast, once combined to win 11 straight.

The unbreakable grip of the top players on the men’s game didn’t just rewrite the record books; over time, it changed the way we thought about men’s tennis. The cast of characters got smaller but also more vivid. Tournaments stopped seeming like chaotic assemblages of far-flung talent and became gladiatorial chamber dramas, micro-epics in which the only conflict that really mattered was within the tiny field of potential champions. Ten days of orderly prologue would produce two or three matches of stupendous significance, matches that more often than not outshone their own blazing hype. Winning a Grand Slam tournament no longer seemed like an unlikely thing for anyone to do. It seemed like an expected thing for a few people to do, and an impossible thing for everyone else.

Thomas Cole, New-York Historical Society

This state of things seemed, from a certain angle, like it might go on forever. Federer’s gentle decline from about 2010 on was mirrored by Djokovic’s rise; Nadal’s periodic tendency to break down in a heap of rusty bolts and carburetor parts was matched by Murray’s arrhythmic access to genius. The cosmic order rolled on. Only then last year happened. First Stan Wawrinka — playing like John McEnroe inside a longshoreman Jaeger — went through both Djokovic and Nadal to win the Australian Open, becoming the first non–Big Four player since del Potro to win a major. Then Nadal suffered a string of nagging injuries, missed time, and lost to Australian teenager Nick Kyrgios at Wimbledon. Murray spent the year in a mauve funk, parting ways with his coach, Ivan Lendl, and not beating another top-10 player until the U.S. Open. Federer played well all year, but in classic late-Federer style, “well” meant “as brilliantly as possible without becoming a credible threat at any major except Wimbledon.” Djokovic won the year by default, but he couldn’t get past Kei Nishikori in the U.S. Open semifinals; Marin Cilic won that tournament, making 2014 the first year since ’03 to see two first-time major winners.

The truth is that tennis fans had been murmuring about the end of the Big Four for a while; the hard numbers simply followed a feeling of ambient decline. Partly this is because “the Big Four” is a suspiciously elastic concept, and one that became a little less impressive every time we allowed it to expand. As great as he is, and as unfair as this is, it’s always been hard to escape the sense that Djokovic will never quite do anything to match the drama of the Federer-Nadal rivalry he interrupted. And Murray — well, Murray has won only two majors, and if it weren’t for his British-male-Wimbledon-winner auto-celebrity, he might seem more closely aligned with the Cilic-Wawrinka camp than with the superstars in the former Big Three. Partly the sense of decline was simply the result of Federer’s aging. He’d been the player who shattered your idea of what was possible in tennis, the player who woke the era’s wonder up. Whatever came after him was always going to be an anticlimax, even to fans who spent his whole career rooting against him.

This year’s Australian Open featured a men’s semifinal lineup of Djokovic, Murray (who’s playing brilliantly), the no. 7–seeded Czech Tomas Berdych (who lost a place in the final to Murray in a punishing four-set match last night), and Wawrinka. The last time neither Federer nor Nadal made an Australian Open semifinal, Andre Agassi won the tournament. You can never identify the exact moment when the empire falls; you can only name the moment when you realized it had already fallen. For me, the moment came earlier this week while I watched a passive, rusty, maybe-kinda-injured Nadal lose to Berdych 6-0 in the second set of his straight-sets quarterfinal loss. I was sure he would come back, because if Rafa does anything reliably, it’s claw his way up from a cliff, but not this time. Berdych floated and danced and quietly exploited Nadal’s positioning with wide ad-court shots. Rafa looked wrong-footed and out of ideas, his usual surging energy crumpled into a feral question mark. He looked, frankly, like he’d rather be someplace else, and my brain went: Oh, here we are in the future. I was wondering when that would get here.

Nadal had previously beaten Berdych 17 straight times; his loss came just a few days after Federer dropped his third-round match to Andreas Seppi, whom he had previously beaten 10 straight times. That hollow boom you hear by the gate? That’s the Visigoths knocking.

Thomas Cole, New-York Historical Society

I want to be clear about what I mean by this. It’s not that I thought 2006 was in effect till Monday night, or that I thought Federer and Nadal were still the two best players in the game. Like every other tennis fan, I knew that things had changed, that the walls of the hierarchy were cracking. But the idea of the Big Four, in subtle ways, still directed how I thought about men’s tennis. When I looked at a tournament draw, those names lit up a little brighter than all the others, regardless of where they were seeded. When I weighed championship odds, those were the players I favored, regardless of how anyone else was playing. The Big Four concept was never scientific, but it was never meant to be science. It was a kind of folkloric context, a grammar that organized your impression of the game. And it worked that way for a long time, even after you started to sense that it was slipping, even after you realized you had to overlook important details to make it apply. You didn’t do this consciously. You did it by instinct. You did it because it had worked for so long.

What I realized when Nadal lost is that it doesn’t work anymore. Murray or Djokovic will probably win this tournament; no one is playing better tennis. But empires don’t fall all at once, and they don’t vanish just because they’ve fallen. They break apart into smaller states. Sometimes they form other empires. The Big Four idea has lost its residual validity because it no longer describes, or even really approximates, what you can expect to find when you tune into an ATP tournament. The warring-gods narrative has given way to other stories. Nick Kyrgios is — by a lot, and for many reasons — the most electrifying player on tour. Wawrinka and Djokovic, who play in tonight’s semifinal — which, hoo boy, you should watch — are more likely to play an amazing match against each other than Federer and Nadal. Also by a lot, also for many reasons.

There are no obvious heirs to the gaudy trophy counts and absurd winning streaks of the old guard. But for the first time in years there are openings to exploit, opportunities to seize. It no longer seems impossible, or even especially unlikely, for a major to have a surprise winner. Marin Cilic won the U.S. Open! The chaos inherent to a tennis tournament — where you take the best several dozen players on earth, fly them to an arbitrary location, and pit them against each other for two straight weeks, with every permutation of mood and accident and bad room service and sudden inspiration that implies — is on the rise, after having been all but smoothed out of the game.

We need a new grammar to capture the feeling of this moment, when — for the first time in a decade — no one knows exactly what to expect. It’s uncertain and confusing and in some ways it’s sad, but it’s also thrilling, because it’s full of possibility. That mountain range is now another country. That invincible army is in retreat. The new rules haven’t been written yet. The new stories haven’t been told. You can feel them pulling themselves up till they’re almost at eye level with the old ones. A player you haven’t heard of today will win a major in the next three years. This is where we are, and there’s no going back. Tear down the marvelous capitals.