Rafael Nadal and Roger Federer have achieved a remarkable piece of history after returning to World No.1 and 2 for the first time since 2011.
Nadal's victory at the US Open on Monday cemented his spot as World No.1, while Federer has now overtaken the injured Andy Murray into second spot.
It's the first time since March 2011 that the tennis legends have occupied the top two spots in the world rankings, and it also sets an incredible age-defying record.
Back where they belong. Image: ATP
At 31 and 36 respectively, Nadal and Federer are the oldest combined No.1 and 2 since rankings were introduced in 1973.
With Federer's victories at the Australian Open and Wimbledon, and Nadal's at the French and US Opens, it's the first time since 1969 that all four grand slams have been won by men in their 30s in one calender year.
If you were also to add up their ages when winning each grand slam, their combined age of 132 surpasses the previous record of 122 in 1972.
The amazing resurgence of Nadal and Federer may offer a sense of comforting familiarity, but does it mask the looming deficit of talent beyond the ATP's ruling elite?
For several years now we've been told to savour the likes of Nadal, Federer, Novak Djokovic and Andy Murray as the clock begins to tick on their glittering careers.
But what will happen when the fantastic four finally vacate the grand slam stage?
And, as it stands, the ATP is facing an epidemic of underachievement from among its up-and-comers.
Such has been the extraordinary consistency and longevity of the big four (make that five, if you count Stan Wawrinka), the best of the rest have barely had a sniff at the top prizes.
Juan Martin del Potro and Marin Cilic are the only other major winners since the former's triumph at the US Open in 2009.
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It had appeared the passage of time and mounting injuries, most recently to Djokovic and Murray, would begin to open the door for others.
But rather than finally herald a long-awaited opportunity, a fit-again Federer and Nadal promptly reasserted their dominance to share this year's slams between them.
Nadal and Federer. Image: Getty
The problem is the younger men who have grown up idolising the modern greats and been comprehensively unable to rise to the challenge of facing them on the biggest stages.
Grigor Dimitrov, optimistically nicknamed 'Baby Fed', is now 26 and has just two slam semi-final appearances to his name.
Dominic Thiem, two years Dimitrov's junior but also two places better off in the rankings at seven, has also managed to reach the last-four of a major on just two occasions, and only at Roland Garros.
Rather than capitalising on their physical prime, Milos Raonic (one slam semi and one slam final) and Kei Nishikori (one slam final) have gone backwards in the rankings of late.
Time remains on his side but, barring a transformation in his attitude, 22-year-old Nick Kyrgios appears unlikely to make the most of his considerable ability, which has yielded just two slam quarters.
Many hopes rest on the slight shoulders of Alexander Zverev, who is up to fourth in the world at the tender age of 20 and won two Masters titles this year.
His slam record leaves much to be desired, with the fourth round at Wimbledon the German's best so far, although he has plenty of time to improve on that.
Joining him as the pick of the ATP's 'Next Generation' is Canada's Denis Shapovalov, who claimed the notable scalp of Nadal in Montreal last month.
Russian trio Andrey Rublev, Karen Khachanov and Daniil Medvedev are also 21 or younger and have offered some indication they may succeed where those before them have failed in unseating the dominant cabal at the top of the pile.
The eventual withdrawal of Federer, Nadal, Djokovic, Murray and Wawrinka from the spotlight is inevitable. But will they be overthrown or abdicate?
Watching meekly from the sidelines as the big guns go out entirely on their own terms would be a sorry state of affairs for those hoping to succeed them in the public's affection.
By contrast, rising to the challenge of dethroning some of the sport's all-time greats could cement the status of a player as a force to be reckoned with for years to come. Time will tell if they are capable of rising to that daunting challenge.