The landscape of men's tennis is undergoing a seismic shift. The 'big four' of Novak Djokovic, Andy Murray, Rafael Nadal and Roger Federer could well be a thing of the past.
After losing the final of the US Open in September, Djokovic claimed his conqueror, Stan Wawrinka, had established a 'big five' by winning his third grand slam title, despite the Swiss' arguments to the contrary.
"He plays best in the big matches and definitely deserves to be mentioned in the mix of top players," said Djokovic.
But as the season-ending ATP World Tour Finals arrive, no longer do a group of four, let alone five, elite players reign supreme.
After completing a career Grand Slam with a long-awaited maiden French Open triumph in June, the previously dominant Djokovic has suffered a nosedive in form and lost his place at the top of the rankings to Murray, previously the weakest member of the so-called big four.
The Serbian had been the world number one since July 2014, but his quarter-final exit at last week's Paris Masters and Murray's run to the final - in which he defeated John Isner - resulted in Djokovic being overthrown. Ahead of that tournament, Djokovic admitted he had been struggling for motivation since his success at Roland Garros.
He explained: "Winning the French Open this year has brought a lot of joy to me but on the other hand has taken away a lot from me, as well. I felt a little bit exhausted, I must say, and maybe less motivated."
Perhaps a lack of challenging opponents contributed to Djokovic's change in fortunes. Federer and Nadal, who boast a remarkable 31 major victories between them, have each been blighted by injuries in 2016.
After a run to the Australian Open semi-finals, Federer, 35, underwent knee surgery and pulled out of the French Open with a back problem. Despite reaching the semi-finals at Wimbledon, he then ended his season early to focus on being fit for next year and now sits 16th in the rankings.
For Nadal, a shock first-round exit in Melbourne was followed by a wrist injury that forced him to withdraw at Roland Garros, and, although he returned to claim Olympic gold in the men's doubles with Marc Lopez and reach the fourth round of the US Open, he too curtailed 2016 prematurely.
It would be foolish to write off players of such proven quality, but is natural to question whether, given their ongoing fitness concerns, they will be able to return to their previous heights.
The result of their absences is arguably one of the weakest fields at the ATP World Tour Finals in recent years, with Djokovic the only previous finalist and winner.
Murray's phenomenal year and improvement since reuniting with coach Ivan Lendl, a relationship which yielded a third grand slam at Wimbledon this year, cannot be swept aside. Yet the fact that the level of top-grade competition has declined this year has no doubt aided his strong showings.
In London, the Briton, who could be displaced at the top of the rankings before the end of the year, has been drawn in the John McEnroe Group alongside Wawrinka, often a formidable opponent but one lacking the on-court temperament of a consistent winner. Marin Cilic and Kei Nishikori, who both struggle to regularly deliver their best tennis, make up the pool.
Djokovic, meanwhile, faces Milos Raonic - the Canadian is hunting down world number three Wawrinka, but injured an ankle in Paris - and debutants Gael Monfils and Dominic Thiem in the Ivan Lendl Group.
The Serbian has won the last four year-ending competitions, and, barring a shock, appears likely to meet Murray - the man he beat to win this year's Australian and French Opens - in the final.
It could well be a regular theme from 2017.