UNIONDALE, N.Y. -- John McEnroe was there to greet Kei Nishikori at the end of the tunnel leading from the court to the backstage area under the Nassau Coliseum. Nishikori had just won his first-round match at the New York Open on Tuesday, his first tour-level encounter since he was forced to the sideline by a severe wrist injury last summer in Montreal.
"It's great to see you back," McEnroe said as Nishikori stopped for a moment on his way to press. "Your forehand looked tremendous tonight. You hit a very sweet ball out there."
Nishikori grinned. McEnroe chuckled, adding that Nishikori ought to come in to net a little more frequently behind some of those atomic groundstrokes. "Yeah, I'm trying," Nishikori said. "I just need to get more matches."
With Nishikori gone, an onlooker asked McEnroe, "Think he'll take that advice?"
"He'd better, or else, forget it, man," McEnroe replied. "He's been No. 4 in the world. But where can he go from here?"
A lot depends on his health. Nishikori has paid an incrementally higher physical price as the years have passed, including that extended wrist injury.
Nishikori, 28, is a former Grand Slam finalist, but he appears mired -- albeit on exceptionally high ground -- in his place in the tennis hierarchy. He has rarely been able to crack the code of the Big Four, who have dominated Grand Slam events for a decade and a half.
No player born in the 1990s has won a Grand Slam event, and that includes Nishikori, who reached the 2014 US Open final. Only four players have been ranked No. 1 on the ATP computer since 2004, and by now, you know who they are.
So many of the talented challengers to Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray have struggled to keep pace, and like Nishikori, they have spent a lot of time on the injured list. It suggests that the group is burning itself out trying to keep up.
"Yeah, I feel the pressure to work harder," Nishikori told ESPN.com on Sunday before the start of the tournament. "But it gives me good motivation. I had many injuries. I have to work a lot more off the court, but I'm going to keep trying."
Milos Raonic, a 27-year-old former Wimbledon finalist who has been ranked as high as No. 3, is in a similar situation. He has played just two matches this year, losing both. He was derailed last summer by leg and wrist injuries. He's down to No. 31 in the rankings.
Grigor Dimitrov, another under-30 player on the short list of competitors once expected to evolve into Grand Slam champions, has yet to make a major final. But at least he has remained healthy and is getting closer.
McEnroe, who coached Raonic to the 2016 Wimbledon final, told ESPN.com, "Milos had kind of the same thing happen as Kei. He pushed it; he wanted desperately to get better. It hasn't been easy for either of them. They're trying desperately to figure out how to win a major, but you have to wonder if they're going to miss out."
That question took on new dimensions early this year because of a dramatic shift in the tennis landscape. The Big Four continued to impersonate tennis supermen right through the middle of last year, but then the underpinnings fell away. Between then and the end of January, Djokovic, Murray, Nadal and even Stan Wawrinka were all laid low by injury.
Suddenly, this appears to be a year of enormous opportunity.
But as Nishikori approaches 30, the toll taken by his style is likely to become more pronounced. He is, of course, injury-prone and down to No. 27 in the rankings, which means the Japanese star will have to put in a lot of time on court, grinding out wins against tougher opponents early on -- a potential physical plight that will no doubt test his rebuilt wrist.
"It's very tough to maintain the balance," Nishikori said of his attempt to get adequate competition, quality rest and high-grade preventative training. "If you're not in the top three or four, you have to play a lot of tournaments. Many guys got injured last year, but you have to accept that and keep working on your physical [fitness] every week."
Nishikori's reluctance to embrace a more aggressive style hurt him in his battles against the Big Four. But given the compromised state of the ruling quartet, he just might be able to spin his wheels free and find the form that led him to a near title in New York nearly four years ago.
"Take Federer out of equation, and it's unpredictable," McEnroe said. "Somebody has to tell these guys there's this opening, and it's been as big as it's been in 10 years."
Nishikori is listening. It's just a matter of whether his body is as well.