Watching the resurgent Rafael Nadal add another US Open title to his collection offered us all a sense of comforting familiarity, but should not serve to 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, Roger Federer, Novak Djokovic and Andy Murray as the clock begins to tick on their glittering careers. 

After all, they can't rule the roost at the top of the men's game indefinitely. 

But what will happen when the fantastic four finally vacate the grand slam stage?

Those anointed as the sport's future stars are consistently failing to seize the mantle offered to them.

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.

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.

After beating Sam Querrey to book a place in the semi-finals at Flushing Meadows last week, Kevin Anderson summed up the situation that has faced his fellow long-term members of the top 30.

"We are so accustomed to it, a decade, the same guys being there," he said of the game's elite.

Anderson lost the final in straight sets to Nadal at Arthur Ashe Stadium, but it is not the 31-year-old South African's fate that observers, organisers and sponsors have been increasingly despairing of. 

Rather, 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.

A glance at the upper reaches of the rankings throws up several repeat offenders.

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.