If it were possible to get into someone's head, most tennis players probably wouldn't mind touring the brain of Serena Williams, with the first stop being whichever region houses that famous switch she somehow flips with impunity whenever she needs to pull out a match.
Until then, even greats like Chris Evert and John McEnroe rally theories back and forth with no more handle on the phenomenon, and how she does it, than the rest of us.
McEnroe: "I wish I knew the answer to that because I would've done that more. To me, she's the greatest. I've never seen someone come back from behind as much as she has."
Evert: "I mean, how she digs herself out of holes ... I think she gets mad at herself, and that's the motivation and that gets her going."
McEnroe: "If getting mad was the criteria for coming back, I would have been the all-time Grand Slam men's singles leader. So it's got to be a little more than that."
Indeed, as Williams goes for her 21st major title this Wimbledon fortnight, a feat that would pull her to within one of Steffi Graf for the most in the Open era while keeping alive hopes for her first career calendar-year Grand Slam, her ability to turn seemingly losing efforts in her favor has become still another Serena trademark.
As dominant as she has been, capturing five of her Slam titles without dropping a set, Williams, who is three months shy of her 34th birthday, also has this amazing statistic attached to her: She has bounced back from a set down to win 33 Grand Slam matches. And since her surprising third-round exit in last year's Wimbledon singles, Williams has come back 13 times to win matches in which she lost the first set. Six of those victories came in the Australian and French Opens, her 19th and 20th Grand Slam singles titles.
Wiping away memories of a bizarre exit from Wimbledon last year, when she melted down physically in doubles, Williams has established a new kind of dominance. And one that, in its own way, is just as impressive.
"That tells you something about how incredible she is," McEnroe said.
At the French Open, Williams dropped the first set in four of her seven matches en route to the title. She was down a set and trailing 4-2 against Victoria Azarenka in a third-round match before pulling out a 3-6, 6-4, 6-2 victory.
"That was the best I've seen [Azarenka] play since she got hurt [in 2014], by far," McEnroe said. "That was 6-4, 4-2 and it was looking dicey, and Serena finds that extra gear, and that's not easy to do, especially when you're not playing that well.
"It's one thing when you're playing some unknown German player [Anna-Lena Friedsam, whom Williams defeated in the second round after losing the first set], but it's another thing when you're playing someone who's the former No. 1 player in the world and has won a couple of Slams and is hungry. And she still did it."
In the round of 16, Williams again lost the first set and was trailing 5-4 to Sloane Stephens (who was twice within three points of victory) before pulling out a 1-6, 7-5, 6-3 win.
A clearly ailing Williams also defeated Timea Bacsinszky in the semifinals 4-6, 6-3, 6-0 after being down a break in the second set, then ripping off the final 10 games of the match.
"I think at the French, you had two things going on," Evert said. "She was starting out slowly, and at the beginning of the week she would go out and think that cruising along was good enough, and she got herself into trouble.
"Later on in the tournament, she was definitely ill with a cough, and she probably even had a fever. I think that slowed her down a little bit."
But more to the point, Evert said, is that it simply becomes increasingly difficult for players in their 30s to maintain their intensity and consistency.
"I know this from my own experience," she said. "My last US Open I played probably one of the best tennis matches in my life beating Monica Seles. ... And then in the next round, I lost to Zina Garrison, who I owned. I never lost to Zina Garrison. I was flat.
"So I think sometimes [Serena] comes out flat. Sometimes she comes out maybe not 100 percent fully engaged, like she kind of cruises along and then maybe it hits her, her pride [telling her], 'I don't want to lose this match' ... and she'll start slapping some winners and she'll give you that scream and then she's won the match."
Evert laughed at the absurdity of that skill, McEnroe calling the belief Williams has in herself to win "a gift." But is there a psychology behind it?
Jim Loehr, a performance psychologist who has worked with a number of top tennis players, said he believes Williams has developed a pattern of getting herself into competitive holes spurred on in part by not meeting the high standards she sets for herself. Then nerves can temporarily take over.
"She often waits for something to happen to lift her spirits and then all of a sudden, it's game on," Loehr said.
Patrick Cohn, a mental game coach and owner of Florida-based Peak Performance Sports, said that for Williams, falling behind in matches may have a subconscious benefit: helping her conserve emotional and physical energy. Over the course of a tournament, that helps her.
Working primarily with top juniors as well as high school and college tennis players, Cohn said his biggest challenge is persuading young players not to give in to negative emotions and essentially tank a match.
But for Williams, a history of repeatedly overcoming slow starts has only made her more confident and more dangerous, while opponents also familiar with the Serena phenomenon tend to retreat at the first sign of another famous comeback.
Evert referred to "that scream," saying, "She's got to get herself going," and it is as striking as the familiar loping walk between points and the negative body language that usually accompanies Williams' slow starts.
After dropping the second set and falling behind 2-0 in the third in her French Open final to Lucie Safarova, Williams engaged in a shouting session with herself before taking the last six games and the title.
"The thing you notice about Serena is she's so demonstrative about it," said Billie Jean King, winner of 12 Grand Slam singles titles and 39 major titles overall. "Her body language is so strong. ... Serena is not subtle. She's very dramatic. She hates to lose. I think she hates to lose as much as anybody, which is why she's so great."
King said it's Williams' serve, arguably the biggest weapon in all of tennis, that most often allows her to "storm back into matches."
"Other players don't have that gift," King said. "She'll be behind and she can serve two or three aces in a row. It's very uplifting when you can do that -- they're free points. That lifts you -- mentally, emotionally -- now you can go up another level again. You can breathe."
Whatever that one shot or one point is, it is easy to detect when Williams has flipped the proverbial switch.
"All of a sudden she walks differently, she acts differently and her opponent knows it's over," Loehr said.
What is clearly not over yet is a 20-year pro career that seems, at times, to be only improving as Williams gets older; a fourth straight Grand Slam title and second "Serena Slam" are on the line at the All England Club.
"What people take for granted is that she's almost 34 years old and we haven't seen anything like this in tennis and she shows no sign of slowing up," said Brad Gilbert, a former top-10 player who coached such champions as Andy Murray, Andy Roddick and Andre Agassi.
"It's almost nitpicking to say, 'She won but had to struggle to do it,' because more often than not, she does it so easily that it's even more impressive."
Gilbert said that Williams' ability to manage and escape the most difficult of situations is what has her in rare company and added that "she is only getting stronger."
"Late in her career," McEnroe said, "Serena seems to be mentally tougher than she has ever been. And she has been awfully tough at times."