By Abhishek Mehrotra
On Sunday earlier this week, Nishikori and Raonic faced off in the final of the Rakuten Japan Open. For those two hours and eight minutes at least, it seemed the future of tennis had finally arrived.
Both players had notched up impressive victories on their way to the final. Nishikori, who had last month spoken to ESPNSTAR.com about improving his consistency, beat world number seven Tomas Berydch in straight-sets in the quarters.
Raonic, built like a tank and possessing the firepower of one, notched an even more impressive scalp in the semis - US Open champion and the man who had so beautifully dismantled him in straight sets at Flushing Meadows - Andy Murray.
You wouldn't equate a couple of encouraging results with the dawn of a new age in tennis, not with the most dominant top four tennis has ever seen still around, but maybe the land of the rising sun had cast a few slivers of light on who our next champions would be? Not so. At least not on the evidence of the past couple of days.
Nishikori fell in three sets to Sam Querrey in the second round, Raonic in three tie-breakers to Marcos Baghdatis at the same stage.
At the US Open last month, I spoke to Steve Tignor, author of High Strung - a riveting and suitably frenetic account of the wild wild west tennis era of the 70s and early 80s - and a journalist who has covered the sport for more than a decade, about how it's the older players who're dominating at the very top level.
"I was watching a qualifying match - Ryan Harrison's younger brother Christian [who's 18 - just a year younger to Pete Sampras when the latter won the US Open in 1990], and you could see the difference physically between him and the player he was playing - a solid rank and file player.
"He just hit the ball too hard for him. Too heavy, too deep. I think at all levels of the men's field - they can hit the ball heavy. That wasn't always the case. [Now] it's hard for a young player. You need to be a little stronger, need to have developed a little more.
"At the same time, that's true until the next Nadal comes along. It's true for right now but I think there will be another Nadal who wins a Grand Slam, who's physically ready and blasts through people."
Janko Tipsarevic, the 28-year-old world number nine, who broke into the top 10 just a year ago, had this to say when I put the same question to him.
"Tennis has become more about fitness than ever before. So the older guys like me who have experience of how it is - not all of us are like Nadal or Djokovic who came into the top five at age 20 and stayed there.
"So the players at the top - Berdych , me, Tsonga  - we realise how good our positions are right now - we're trying not to let it go. Trying to stay injury free."
Between them, Tignor and Tipsarevic's explanations can perhaps account partly for the inconsistency of Nishikori and Raonic even though it is hard to see Raonic being able to hit any heavier. Already, his serves and groundstrokes thunder like "the crack of doom", to borrow a phrase from English cricket legend Wally Hammond.
However, while Raonic and Nishikori have undoubtedly made some sort of progress, Tomic, the 19-year-old Australian - touted as the next big thing in tennis as recently as early this year when he reached the fourth round of the Australian Open - has seen his career go in the opposite direction.
Tomic failed to pass the third round in any of the three remaining Grand Slams or the eight ATP Masters 1000 tournaments he participated in this year. The horror run that will soon see him drop out of the world's top 50 included a 6-4 6-0 loss to Florian Mayer in the first round in Shanghai.
When he lost the third set 6-0 to Andy Roddick at the US Open in September, his performance, so listless that John McEnroe wondered aloud on television if this was a tank job, was dubbed "disgraceful" by Patrick Rafter.
Clearly, mental issues abound here. It would be a pity if one of the most crafty and stylistically unique players of the current crop were to waste his potential. The sport has seen plenty of those.
Perhaps it's too early to pass judgement on these youngsters.
Maybe the likes of Raonic, Nishikori and Tomic will make their big breakthrough in the coming months - and stake a serious claim to taking over the baton from the sport's current superstars. Maybe an unheralded youngster will hold the sport in thrall next year.
It's a "maybe" that hovers uncomfortably over the game at the moment, churlish though that may sound given the "golden" tag that this era justifiably sports. But, such has been their brilliance, and sustained for so long, that it's easy to forget how young each of the top four were when they first burst on to the scene.
Federer was 21 when he won Wimbledon in 2003. Djokovic 20 when he took the Australian Open in 2008 and Nadal 19 when he won the French Open in 2005. Even Murray, who won his first Slam at 25, reached his maiden final at 21 and took his first Masters title at the same age.
None of the current crop has come anywhere near replicating these achievements.
"The guys I like of the youngsters are Tomic, [Grigor] Dmitrov, [Alexandr] Dolgopolov. Just because they're great to watch, but I don't really count on any of those guys going on win a Grand Slam. So there's still a bit of a question mark as to the next generation," said Tignor.
As another tennis season edges to its climax with Federer, Djokovic, Nadal and Murray having swept every single Grand Slam and all seven of the nine Masters tournaments played so far - that question continues to make its presence felt in the background with increasing intensity.