With Serena Williams attempting to land a 24th Grand Slam singles title, her former hitting-partner and confidante Sascha Bajin gives an insight into the American's world.
Williams won 10 Grand Slams during her eight years with Bajin, who also coached Naomi Osaka to two majors and the world No.1 ranking.
Opponents should be worried if Williams suddenly stops grunting and goes completely quiet.
"When Serena has an important moment, she’ll go very quiet, and go into herself somehow, and that’s when she is at her most dangerous, not when she’s grunting loudly," Bajin writes in his new book, 'Strengthen Your Mind: 50 Habits for Mental Change', which is published in Japanese this week.
"She’s totally in the zone and trying to find that perfect mindset where she can fight through anything."
Williams has her fears, on and off the court.
"The truth is, everyone's afraid. If it feels as though you are paralysed by fear, with your worries and rising panic holding you back in your life, it could help you to know that every human being is scared of something," Bajin writes.
"Fame and money won't protect you. From working with Serena for many years, I know she also has her fears. She has always had her fears on court, and now she is also scared of not being a good mother."
Williams wins matches through her body language.
"Serena carries herself on court as if she's the best in the world. When Serena takes the first set, she makes her opponent feel as though there’s no way on earth that you’re going to beat her.
She does that through her body language, through her cries of 'C'mon' and the way she's in an opponent's face.
She’s on to you. She’s showing you her strength by the way she talks and the way she walks."
Williams likes to isolate herself before big matches.
"In the hours and then minutes before a big moment, some people like to shut themselves away from everyone; they don’t want to talk and they don’t want others to speak to them," Bajin writes.
"When I was with Serena, she used to get very quiet before matches, as she was completely locked-in. You had to walk on eggshells around her, to make sure everything was right, and that nothing would make her lose that focus."
Williams is "bullied" by trolls.
"Even Serena has been bullied. You can be sure she would have received a bunch of messages after the 2018 US Open, for losing that match.
And those wouldn't have been messages that she would have wanted to read," Bajin writes. "I received death threats before the 2018 US Open final [between Bajin's then player, Osaka, and Williams]. I was also called a traitor. I was told that I was a loser, that I was nothing, a nobody.
"My social media was blowing up because I had worked for Serena for so many years. Those threats and insults came on social media from a small number of Serena's fans, who were angry that, as Naomi's coach, I would be doing whatever I could to stop Serena winning the title.
Those fans somehow saw my new role as a betrayal and warned me: 'You'd better not give away Serena's secrets.' That was bullying. Those fans were trying to intimidate and bully me before the most important occasion of my tennis life, and I wasn't going to allow it."
Williams is superstitious by nature.
"During a tournament, she always uses the same shower and the same locker.
Another of her superstitions is what she does after the first game of the match - she doesn't stop by her chair and take a sip of her drink, as most players tend to, but walks around the net on the other side."
Williams and Bajin were "so close".
"For the first few years after moving to the States, I even lived with her in Los Angeles and Florida. Most of my memories of my years with Serena are happy ones.
I have a painting hanging on the wall of my kitchen, which was a gift from Serena, and reminds me of the amazing times we had together.
The picture is of Serena on court, and includes a message from her: 'To Sascha, my inspiration, my brother, my love, my family. I will love you for ever and ever. Serena.’ I like looking at that painting, while I also have some of her old rackets in my garage, including a few that she smashed on court."
Williams believes in mind over matter.
"Serena said to me once that she couldn’t understand people who can’t quit smoking or drinking. I said that’s tough because people are addicted to it. They crave the nicotine or the alcohol.
She replied that the beautiful thing about the mind is that if you tell yourself something you can immediately change your life; it doesn’t have to take a long time to address your problems," Bajin writes.
"If the body has a bad habit, Serena said, it should be possible to address that habit with a strong mind. It just comes down to how much you truly want to make changes. That had an impact on me: I learned that you can make immediate and lasting change."
Unlike most other players, Williams doesn't ever get bored on the practice court.
"Boredom is a problem for almost everyone at work. Even elite tennis players occasionally feel bored at work. That's inevitable when you're doing the same thing again and again, whether on the practice court or in the office. Your mind tires of the repetition, and you can feel your energy and enthusiasm draining away.
About the only exception is Serena, who is capable of doing the same thing day in and day out without getting bored; that girl's a machine."
Williams is completely open with those she works with.
"Trust has to be earned. In a player-coach relationship, and also in any key business relationship, the fastest way of earning trust is to be completely open and honest with someone - when I was working with Serena, she knew all my secrets and I knew all hers.
Serena knew more about me than my own mother did."