Big Four may now be one (Djokovic) and done (Murray, Nadal and Federer)

Novak Djokovic, sovereign again, experienced an unexpected crisis on the very last day of the ATP tennis season on Sunday. He was beaten in the ATP Finals by 21-year-old Alexander Zverev. But don't read too much into it. Djokovic will go into the new year the prohibitive favorite to dominate in 2019.

The obvious reason is Djokovic's furious resurgence in the second half of the year: the sustained excellence that produced wins in 35 of his last 38 matches and four titles (including two majors) in 2018. The less obvious factor in his rosy outlook is the sense that a long anticipated changing of the guard at the ATP is accelerating, lacking only one or two sensational results to make it a runaway story.

That's the main theme woven into the takeaways from the 2018 ATP season:

The Big Four may be down to a Big One

Tennis historians may look back upon the 2018 US Open as a tipping point in the history of the quartet that once ruled tennis. The Open was one of the increasingly rare majors when all four were entered in the same major again.

Djokovic won it, but this time he didn't have to beat any of his vaunted rivals. Federer lost to an Australian journeyman in fourth round, Andy Murray (recovering from hip surgery) was knocked out in the second round by 35-year-old No. 31-ranked Fernando Verdasco, Nadal retired during his semifinal against Juan Martin del Potro with a muscle pull.

Ominous signs: Nadal never played another match in 2018, and he underwent ankle surgery in the fall. Murray's recovery has been plagued by setbacks. Federer, despite clinging to a No. 3 ranking, was 4-6 against Top 10 opposition in 2018 -- and that includes a loss to a slumping Grigor Dimitrov.

The sky's the limit

Diego Schwartzman, the 5-foot-7 dynamo from Argentina, had a career year in 2018, hitting a career high of No. 11 (finishing at No. 17). But Diego probably would be happy to change bodies with his countryman del Potro. The 6-foot-6 star also had a career year, peaking at No. 3 before he was laid low by injury and forced to miss the ATP Finals.

The reality is that size matters. It seems to matter more with each passing year. Zverev and fellow ATP Finals qualifier Marin Cilic also are 6-foot-6, while Wimbledon finalist Kevin Anderson is 6-foot-8. John Isner, who made the ATP Finals for the first time in his career this year is 2 inches taller than Anderson. Only Kei Nishikori among the elite eight in London was under 6-foot-1.

This isn't coincidence -- it's a clear trend. According to an analysis posted on Twitter by @VoodeMar, the average height of the year-end qualifiers in 1978 was 5-foot-10. Over the next two decades it rose to 6-foot-1. In 2008 it gained another inch. This year, the average height was 6-foot-4.

The #NextgenATP Barbarians are at the Gate

It seemed fitting. Karen Khachanov, 22, won a breakthrough Paris Masters, and barely a week later Zverev chose to skip the #NextgenATP exhibition in Milan and won the ATP Finals in London.

That was two massive blows struck against the empire of 30-something players who have lately held the ATP in lockdown. And more young hellions wait in the wings. Five of the six players ranked between No. 11 and 16 are between the ages of 23 and 20. That includes Stefanos Tsitsipas, the 20-year-old who rose from No. 91 to No. 15 this year. Others, including Alex De Minaur,Taylor Fritz, Frances Tiafoe and Denis Shapovalov, have had outstanding seasons. But Zverev and Khachanov are drawing the most notice.

Craig O'Shannessy, an ATP analyst and adviser to Djokovic, told ESPN.com: "I see Khachanov as a top-five guy. His attitude is ridiculously good, and he's excellent at the net. He knows how to use the angles and hardly misses a volley."

Storm clouds are on the horizon

Tennis has enjoyed a state of relative peace for some years now. But the turbulence in the rankings is mirrored in the infrastructure.

The year started with Djokovic ordering officials of the ATP out of the room during a player meeting at the Australian Open, so that the players could hear legal advice about their rights to strike.

The majors continued to grow in prestige and strength, with the players continuing to express a desire for a larger share. Federer recently told the Times of London. "The players make 8 percent [of the total revenue]. It's not like we're saying £2m (roughly $2.5 million) for the winner isn't enough. We're arguing that the first-round qualifier only makes maybe a few thousand. We're talking about making sure that, at the lower level, more people can survive."

Sensing the opportunity created by ITF's decision to abandon Davis Cup as we've known it for 100-plus years, the ATP created its own "World Cup" team event, scheduled to start in 2020. The re-imagined and already much criticized "new" Davis Cup will launch next year in Spain. Tennis Australia, that nation's equivalent to the USTA, has become aggressively entrepreneurial, heavily investing in -- among other things -- the Laver Cup. People are bound to step on each other's toes.

Team events are the new frontier

The Laver Cup, the brainchild of Roger Federer, established the gold standard for team events with its inaugural event in Prague in 2017. This year's event, in Chicago, was even better.

Players, it turns out, simply love being on a team. They especially love it in the fall, after the final Grand Slam season is done. The camaraderie and challenge of team play inspires the players and, perhaps most of all, nobody wants to let down the side or look bad in front of his peers.

When John Isner of Team World was asked how "seriously" he took the Laver Cup assignment, he replied: "Honestly, that question really annoys me. One hundred percent serious. This is not an exhibition at all. Not at all. One hundred percent."

Don't write off Federer just yet

Yes, it's Djokovic's world now. Some aspects of Federer's year were sobering, if not exactly alarming. He seemed to run out of steam in the second half. But here's the thing: Federer is an amazing specimen. Nadal and Murray are huge question marks because of injuries and surgeries. Federer had letdowns that may have been age-related, but he bounced right back, right up that great performance in his penultimate match, the payback win over Anderson.

It's all because Federer has a secret, besides his health and fitness. He told reporters at the end of his run in London: "I must tell you I'm very proud that at 37 I'm still so competitive and so happy playing tennis. From that standpoint, I'm actually very happy about the season."

He may have his hands full with Djokovic, he may take the unexpected loss here and there, or bungle big opportunities that he used to secure back in the day. But he loves playing, and that's becoming one of his most formidable weapons. As long as Federer shows up in 2019, anything may happen.