As great athletes age, the first thing we look for is evidence of physical decline. The cliche is to talk about the 'half-step' that has been lost, rather than their psychological state.
Yet even sporting legends can be worn down mentally by the accretion of small failures over the years and the fear that certain opportunities may never come around again. Grey matter is as susceptible to the march of time as muscle fibre.
At 28, Rafael Nadal still has plenty of mileage left in his legs, but the Spaniard admitted after his third-round defeat in Miami on Sunday night that he had been experiencing a new sensation: lengthy spells of nervous tension on the court.
"It is not a question of tennis," Nadal said after his 6-4, 2-6, 6-3 defeat by his compatriot Fernando Verdasco. "The thing is the question of being relaxed enough to play well.
"A month and a half ago, I didn't have the game. My game has improved but I am still playing with too much nerves for a lot of important moments."
There was air of perplexity about Nadal, a sense of a man thinking aloud as he chewed over these unfamiliar issues. For we are talking about a player routinely described as the greatest competitor in the history of the game.
His mental steel was forged by his uncle Toni, who has pushed him to his limits since he was big enough to hold a racket.
His greatest asset, even more than his lassoing forehand, has always been his ability to play every rally as if it were championship point at Roland Garros.
Yet, since he returned to the court at the beginning of the year after a season that was disrupted by back trouble, wrist injuries and appendicitis, the Nadal we know has endured a reverse phone-box transformation, switching from a tennis superman back into Clark Kent.
His defeat by Verdasco, a man he had beaten on their previous 13 meetings, was his fifth loss of the season, and none of those has come against one of his leading rivals. The last time he played another member of the so-called 'Big Four' was in the French Open final last year.
"I am feeling more tired than usual," said Nadal (above), who will now slip back a minimum of one place from his ranking of No 3.
"Feeling that I don't have this self-confidence that I am going to hit the ball where I want to hit the ball, to go for the ball knowing that my position will be the right one.
"All these are small things that are difficult to explain. One of the tougher things has been fixed - that is the game. Now I need to fix the nerves, the self-control on court."
There is an instructive parallel here with Roger Federer. In 2013, at the age of 31, the most successful player of this millennium went through a miserable period where he, too, was losing to opponents he would normally have crucified - Sergiy Stakhovsky, Federico Delbonis and Daniel Brands in successive tournaments.
"Last year, I lost my confidence," Federer later told Sports Illustrated magazine. "Instead of serving it out, you won't. Or instead of making that break point, you won't."
There is a contrast between the way that these two giants approach their press conferences. Federer is a natural poker player, a man who will only ever admit his problems in retrospect, whereas Nadal is a compulsive sharer, forever talking his prospects down to take the pressure off himself.
On the court, though, both suffered the same symptom when their confidence declined: an inability to produce their best on the biggest points.
And yet the process is reversible - as Federer's renaissance has demonstrated.
Nadal will surely find that his own self-belief comes rushing back when his feet touch the red dirt of Monte Carlo - a tournament he has won eight times.
But even if he does recapture his best during the European clay-court season, you wonder whether his body and mind can still cope with the rest of the sport's gruelling calendar.
"Rafa is no ordinary 28-year-old," ESPN analyst Brad Gilbert said yesterday. "He has played a ton of tennis, and suffered a ton of injuries - 10 times more than Roger.
"All the expectation was that he would come roaring back from this last set of problems like he did in 2013, after being out for eight months because of his knees. But it hasn't happened and it will be fascinating to see where his season goes next." (© Daily Telegraph, London)