It's not as easy to distinguish as a trainer-taped shoulder or a knee that clearly isn't working properly. Usually, you can see it in moments of crisis.
The silent assassin.
Nadal has enjoyed great success at the Australian Open. He won it in 2009 and reached last year's final against Stan Wawrinka. But this year he's fighting a flurry of uncharacteristically negative thoughts. The rusty No. 3 seed was solid enough in his opening victory over Mikhail Youzhny, dropping only seven games, but against American qualifier Tim Smyczek in Rod Laver Arena on Wednesday night, Rafa looked genuinely out of sorts.
It was evident in his always-expressive brown eyes. And some obvious and ongoing stomach distress throughout the match.
The 28-year-old Spaniard was a game point from an 0-3 third-set deficit, and even after he managed to break Smyczek, Nadal was greeted by the doctor and trainer during the changeover. After swallowing a pill, Nadal returned to the court.
Whatever it was, it wasn't bitter.
In the end, a weary, exceedingly grim Nadal prevailed 6-2, 3-6, 6-7 (2), 6-3, 7-5 in a gut-wrenching match for both players and all who watched. When it was over, 4 hours, 12 minutes after it started, Nadal collapsed to his knees as if he had won another French Open.
"Very tough night for me," Nadal said in his on-court interview. "Not a lot of people think they will see this, 6-5 in the fifth."
Nadal said he felt tired after the first set and cramps "around the body in different places. All this process is normal when you are not on the tour for so long.
"Hopefully, I have the chance to be back at my level soon."
Before the tournament, Rafa saw this coming. He acknowledged the culprit: his injury-strewn 2014 season.
"Always you need to play more matches than four or five in seven months," he said. "That's a thing that everybody knows. Every time you come back you have the doubts, you have the feeling that you are far from your best."
His knees have always been chronically fragile, but last year his right wrist joined the list of endangered joints and his back proved to be an even larger problem. A late-season appendectomy didn't help speed his rehabilitation. In all, he played only seven matches after a shocking loss to Australian teenager Nick Kyrgios at Wimbledon.
Asked to name the tournament favorites, Nadal said: Novak, Roger and Andy. What about Rafa?
"No, I don't consider myself one of the favorites here," he said. "Last year, yes. This year is a different story. Would be lying if I say I feel that I am ready to win today."
No, he wasn't lying.
The first set went well, but in the second, Nadal started breaking badly. He hit all of two winners -- against 14 unforced errors. His serve virtually disappeared; Rafa won only 10 of the 24 points on his racket and Smyczek converted all three of his break opportunities in the second set.
When he went down an early break in the third, Nadal suddenly looked terribly vulnerable. In his terrific career that has netted him 14 Grand Slam singles titles, Rafa had never, ever lost to a qualifier at a major. Now, it seemed to be a real possibility.
But great players often rally when they're not at their best -- that's why they're great. Earlier in the day, Maria Sharapova came back from the brink (1-4, 15-40 in the third) to defeat Alexandra Panova, a 25-year-old Russian qualifier who had never won a major match before this fortnight. Roger Federer actually lost his first set to Simone Bolelli before collecting himself and winning the last three.
When Nadal broke Smyczek at 3-all, thanks to a botched easy shot at net, common sense suggested it was over. But Smyczek, a 27-year-old from Milwaukee who lives in Tampa, Florida, hung around. He earned two break points on back-to-back Nadal service games and converted the third to level the set at 5-all. The tiebreaker was truly astonishing, as Smyczek sent an ace down the middle to take it 7-2.
When Rafa gets anxious, he retreats deep behind the baseline. That has always been his safe place, but against a fluid, forceful player such as the 5-foot-9, 160-pound Smyczek, it wasn't enough to force the issue. Nadal looked slow and sluggish and couldn't put more than two or three good shots together.
The fourth set, though, was predictable. With Nadal struggling physically and taking inordinate time between points, Smyczek seemed to start feeling the gravity of the moment at hand. The match crossed three hours just before Nadal scored the decisive break that would deliver a fifth set.
Smyczek, game to the end, saved three break points on the way to a 5-all stalemate. But the fourth yielded a tired-looking forehand into the net and Rafa -- after Smyczek bravely saved three match points -- dropped an easy forehand into the open court and then dropped to his knees. It was nearly 11:30 p.m., local time.
Rafa, who plays Dudi Sela on Friday, has reached at least the quarterfinals in each of his past seven trips to Melbourne. Smyczek's best major was the 2013 US Open, where he reached the third round. One day, he'll remember this one more fondly.
In his pre-event news conference, Nadal was asked how close he felt to 100 percent.
"In the end is difficult to say 50 percent, 55 percent, 20 percent," he said. "Doesn't matter. This kind of thing is impossible. Is not mathematics. The only thing I know is I need to work, spend time on court, play matches. I know in terms of being competitive, in terms of rhythm, I will be ready again, no?"
The answer at the moment: not yet.