Imagine the scenario?

A less informed holder of an expensive U.S. Open men's singles final ticket walks into Louis Armstrong Arena, settles down into a court-side seat and then rubs his eyes in disbelief, checking the smallprint on his stub.

"Where's Djokovic? Federer? Thought he'd be here. How about that Scottish dude...Murray. Nadal??? What the heck?!!!"

A showdown between diminutive 24-year-old Japanese Kei Nishikori and giant, bearded Croat Marin Cilic was not what anyone was really expecting, whether it be fans, pundits or TV executives anxiously checking ratings charts.

It was the first grand slam final since 2005 that one of the so-called Fab Four, be it Federer, Nadal, Djokovic or Murray, was not present. Usually it's two of them.

Barring a few small shuffles, the furniture at the top table of the men's game has remained firmly in place for the best part of a decade.

Now, all of a sudden, the rivalries that have become so familiar, be it Nadal v Federer, Djokovic v Murray or Federer v Djokovic are under threat, be it from seasoned campaigners such as Stanislas Wawrinka, rare talents like Grigor Dimitrov or precocious firebrand Nick Kyrgios.

The top guys say they welcome the challenge, but for fans, it will take a little getting used to. For many of them it is like their favorite TV box set is drawing to a close and they are fretting over what they will watch next.

LAMENT BEGINS

Some are already lamenting the beginning of the end of arguably the greatest era in men's tennis - a period which begun in 2003 and has since seen Federer claim 17 grand slam titles, Nadal 14, Djokovic seven and Murray two.

Only six of the 45 majors on offer since Federer's breakthrough moment against Mark Philippoussis 11 years ago have gone elsewhere - two of them this year with Wawrinka beating Nadal in Melbourne and Cilic overpowering Nishikori.

The marketing men at the ATP have had it easy, but a gradual cracking up of the establishment does not mean they'll be hitting the panic button, far from it according to the men's governing body's executive chairman Chris Kermode.

"I think it's an interesting, fascinating time for men's tennis right now," Kermode told Reuters hours before the U.S. Open final. "We have seen a slight shift in the balance this year with new names winning majors.

"We had Wawrinka in Australia, (Ernests) Gulbis beating Federer at the French, Krygios and (Grigor) Dimitrov at Wimbledon and here at the U.S. Open it's been intriguing watching Nishikori and Cilic go through the draw.

"It's a refreshing opportunity for these new guys to be in the limelight and for fans to get to know them more. But I would caveat all of this because I would be careful to write off the top four too quickly, like some are doing.

"Nothing happens overnight and the likes of Roger, Novak, Rafa and Andy won't be holding up the white flag.

"They'll have other ideas. But it does show the depth and talent of the ATP Tour right now."

The men are not alone in exhibiting their strength in depth.

All four women's grand slam crowns were also claimed by four different players in 2014; marking only the third time since tennis turned professional in 1968 that eight different grand slam champions claimed the eight titles on offer.

BATTLE SCARS EVIDENT

While only the foolhardy would bet against a counter-attack from Djokovic and co next season, the battle scars are evident.

Nadal has not struck a ball in anger since his shock defeat by Kyrgios at Wimbledon because of a wrist injury, Murray is without a title of any description since winning Wimbledon in 2013 and recently married Djokovic has gone on record saying his priorities are changing with a baby on the way.

Federer, the oldest of the quartet at 33 and with two sets of twins to raise, actually looks the freshest and the Swiss maestro will move into 2015 believing he can still add to his record grand slam haul.

"It's exciting for the game to see new faces like Kyrgios, now Raonic or Nishikori, you name it," Federer said at Wimbledon this year. "There's been a few guys knocking on the door now.

I think it's good times in tennis right now. There's a lot of excitement. Stadiums are always pretty much full on Centre Court, which is great. I'm really happy playing in this time."

With the big four beginning to creak, any number of candidates are being put forward to fill the void, whether it be Dimitrov, Canadian big server Milos Raonic or, on the evidence of the last few days, Cilic.

Former great John McEnroe believes that far from one or two players hogging the silverware, the next era will see the men's game enter a state of flux.

"I think the next guy who's going to win multiple majors is going to be a guy we don't expect, who we don't know yet," the American said in a recent interview in the Daily Telegraph.

"I'm trying to find out who that guy is, but it's been so difficult for those mid-20s guys to make progress.

While Cilic's win in New York clearly left some in the crowd non-plussed, Kermode says they should not fear.

"The gap is closing," he said. "That's the exciting part, to see which of these young players will continue to break through.

"Roger and Rafa are global icons, not just sporting ones, so that helped (market tennis) but our top-tier events, the Masters 1000 events have got so much bigger that people already know the Raonics, Nishikoris and Dimitrovs.

"This generation coming through have had to live with the realization that they are going to have to beat some of the greatest players ever to play the game so that in itself created a desire and a work ethic that is unprecedented."

(Reporting by Martyn Herman; Editing by Ossian Shine)