It's something that has been thrown around for years, something that is acknowledged yet pushed aside by the top players and something that is potentially happening as we speak.
This sounds like the start of a bad riddle, but just hear me out.
This year's tennis season has seen something absolutely revolutionary in the rise of a lanky 23-year-old Russian by the name of Daniil Medvedev.
Described by tennis analyst Nick Nemeroff as a 'computer-generated player that shouldn't be able to exist', Medvedev started 2019 ranked no. 16 in the world.
ELSEWHERE IN SPORT
He now sits at number four within striking distance of arguably (the arguments against are definitely shocking) the greatest tennis player of all time, Roger Federer.
A key member of the 'don't mention those four words' club, Federer's tennis history is one that I'm not even going to go into detail for as honestly, there isn't enough room here to list his achievements, but one thing you don't want to bring up with ol' Roge is the changing of the guard.
Everyone's favourite American aggressor John McEnroe had a crack at predicting the watershed before the Australian Open which was rapidly blocked by the Swiss superstar.
"I've heard that story the last 10 years. From that standpoint, nothing new there," the 38-year-old said following his loss to rising star Stefanos Tsitsipas in January.
Nothing against the great man but it would be interesting to get his thoughts on that issue later in the season, having tasted defeat to next generation up-and-comers Andrey Rublev, Alexander Zverev, Dominic Thiem and Tsitsipas throughout the season.
I've heard that story the last 10 years. From that standpoint, nothing new thereRoger Federer
Alright, I've gotten semi side-tracked, back to Medvedev, who currently sits with an outstanding win-loss record of 59-17, the most wins of any player on the ATP tour this season.
Far from one of the first names on tennis fans' lips at the beginning of 2019, the Russian has won 11 more matches than Rafael Nadal and Novak Djokovic, while equaling their performance on the titles front with four alongside Thiem.
But it's the recent run of form that has proved truly devastating.
In a feat only ever eclipsed by the big four, which includes the recovering Andy Murray who has just won his first tournament back after featuring in a Partridge opinion column, Medvedev has reached six consecutive ATP finals.
To put that into perspective, the last time he lost in a match that wasn't a final was on July 5, over four months ago in a three-and-a-half hour battle against David Goffin.
So the last time Medvedev was bundled out in an early round, Liam Hemsworth and Miley Cyrus were still married, Dustin Martin was only a one-time Norm Smith Medallist and Theresa May was still Prime Minister of England (in decreasing levels of importance, obviously).
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With next season bound to be Federer's last (but honestly, we've been saying this for years now, the man could continue playing forever for all we know) and his tournament schedule rapidly declining, the results and certainly the ranking, are a chance to slip.
This mightn't create a full paradigm shift but one of the greatest eras in tennis history could come to an end and the big four could be no more, but there are certainly enough players looking to build their own era.
The youngest player in the top 100, Felix Auger-Aliassime is half the age of Federer and not just taking part in the game, he's pushing the game's top players and is ranked 18 having beaten countryman Milos Raonic multiple times.
Most matches played in a career— Mohandas Menon (@mohanstatsman) October 23, 2019
1500 - Roger Federer (won 1232)
1163 - Rafael Nadal (967)
1111 - David Ferrer (734)
1069 - Novak Djokovic (884)
982 - Tomas Berdych (640)
He is joined by fellow Canadian Denis Shapovalov, the 20-year old blonde-haired sensation who sits in 27th position and just took out his first ATP title in Stockholm, Sweden.
Shapovalov leads a progression of seven 20-year-olds sitting in the top 100 including Australians Alex De Minaur and Alexei Popyrin, with De Minaur having claimed three titles this season.
The rise of the sport's future makes for one of the most interesting calendar years in recent memories as the likes of Medvedev look to keep pushing through and truly change the guard.
But just don't mention it to our top three.