The 47-year-old's pulse used to quicken when he would watch Williams, one of history's greatest champions, compete. That was pressure. That was adrenaline. He loved it.
But with his pupil on maternity leave, deep into her third trimester, the coach has to now find the buzz, the rush, from elsewhere, though nothing can truly compare to helping Williams rewrite the record books.
Even live television interviews do not make the heart race, for the Parisian is as comfortable on screen as the rest of us would be sunbathing on a beach.
"You can check my pulse before and after, I am the same," he says, fresh from a live appearance on a British breakfast television programme.
"I find a way to have stress because I need it. I did TV during the French Open and I told the guy 'make me do things I've never done before.'"
This summer marks his fifth as Williams' coach and so, naturally, he has become accustomed to the attention that comes with working alongside the world's most recognizable female athlete, a player who has danced in a Beyonce music video and been a tennis superstar for almost two decades.
Once describing being afraid as a "disaster," Mouratoglou also has no fear, which was a useful trait when confronted by Williams' father, Richard, shortly after the 23-time grand slam champion had appointed the Frenchman as her coach.
Never before had the American worked with someone not sanctioned by her family. The father, the engineer behind Serena and sister Venus' success, needed to be swayed. He was quickly converted and Williams' decision to broaden her horizons proved to be a masterstroke.
Mouratoglou can list Williams' achievements since their partnership began as easily as someone ordering takeaway from a favorite restaurant: 10 grand slam titles, two Olympic golds and a three-and-a-half year stint at the top of the world rankings.
But both he and Williams, he says, are the same characters they were before their alliance.
"I don't think she's changed me or I've changed her," Mouratoglou, who describes himself as a person of extremes, tells CNN.
"I have more exposure clearly than before, but it's also deserved because we did so well.
"When you're with Serena you have a lot of exposure, people look at you different, probably. I'm not saying that they should, but it's a normal process."
'Serena with a baby is still Serena '
It is just before midday on a glorious London summer's day and Mouratoglou has already been interviewed 15 times in the last 24 hours, a number that will only increase in the hours ahead as he promotes his autobiography "The Coach," which is now available in English.
He has had little sleep. His lack of shut-eye, he says, is a combination of boundless media commitments and late-night socializing with friends in England's capital.
He gladly accepts a cup of so-so office-made coffee, although outwardly he doesn't appear to need an injection of caffeine. Wearing a jacket, white shirt and a pocket square, he is the personification of exemplary Gallic tailoring.
And though 30 minutes early for the interview, he doesn't use the spare time to pause. Instead, the loquacious Frenchman is happy to talk at length about Williams -- her pregnancy, her return -- and his rise from an anxious, unhappy child to one of tennis' best coaches.
This is a strange time for the father-of-four. He is at Wimbledon with no student to teach, but this summer, he says, is just the interval before one of the sport's most successful partnerships resumes next year.
Mouratoglou did not know Williams was pregnant during the Australian Open in January, a tournament she won without dropping a set, but the news did not come as a surprise because the player had, he says, been looking forward to becoming a mother "for quite a long time."
Ever since Williams announced in May that she was pregnant, the 35-year-old -- the oldest player to ever be a world No.1, the most relentless accumulator of grand slam titles in nearly 50 years -- has maintained she will play tennis competitively again.
She is a woman who thinks "everything is possible," says Mouratoglou.
"Serena with a baby is still Serena," he continues. "I know she will need time because I know it's a trauma for the body, but we'll take the time.
"It's a new situation but when I see how determined she is at the moment to come back, I'd be really surprised if she changed her mind."
By posting videos on social media of practice sessions at her Miami home, the mum-to-be has been showing the world her steadfast desire to make a comeback and last month flew to Paris to make plans with Mouratoglou for her return.
"She wanted to ask me a lot of things -- what would be the team and when we would start," Mouratoglou explains.
"My job is called adaptation. It's always about adapting to new situations, but the core team is going to be the same. It's just extra people, like a nanny, but even if they come to the court, it's fine, we'll adapt.
"She's incredibly happy and she's frustrated at the same time because she wants to compete.
"She's playing every day. She doesn't move around the court, she just wants to keep contact with the ball and that's it."
'Serena is in a permanent state of non-satisfaction'
It was Williams who, in 2012, picked up the phone and called Mouratoglou to ask if she could use the facilities at his academy on the outskirts of France's capital. He has recently opened another academy in Nice.
Williams had just suffered a surprise first-round defeat at the French Open and two years had passed since she had won a grand slam title. Confidence was low.
Mouratoglou's strategy was to shift this once-in-a-generation talent's focus onto records, onto building a legacy.
He introduced top spin to her forehand, made her play longer rallies and made sure his charge was mentally prepared for games, formulating tactics for each opponent, using his words carefully to inspire.
"It's the little differences," he says. "When players think right it makes a huge difference and, with champions, it's even bigger. She can completely switch from one second to another."
The girl from Compton kept going, even though she was already at an age where many retire, and is now one short of equaling Margaret Court's all-time singles record of 24 grand slams. She is a sporting icon. But what makes Williams a great champion, Mouratoglou explains, is her state of "permanent non-satisfaction."
Victories are forgotten, new goals are quickly set. She works as hard to maintain her greatness as she did to achieve it and, crucially, refuses to accept defeat. This is a player who has won three of her grand slam singles titles after being match point down.
Mouratolgou remembers Williams' victory at the French Open final in 2013, a tournament she had not won for 11 years.
"After the trophy ceremony she looks at me and says 'I want to win Wimbledon,'" he recalls, still impressed by her grit.
"There was zero celebration and, believe me, we tried. We were in the car and she just said 'drive me home.'"
The coach who couldn't connect with people
Mouratoglou is the son of a Greek-born businessman, but despite his family's wealth he remembers his childhood as one of being "paralyzed by the shame of not being able to do better."
An anxious, shy, "puny" kid, his early years were a "disaster," which makes his evolution into an exceptional communicator all the more remarkable.
"I couldn't connect with people so I developed other senses. I had no other option," says Mouratolgou, who grew from an academic under-achiever into a talented singer and jazz guitarist, though tennis was his passion.
"I had to make those changes in order to live the life I wanted to live. Now, from my point of view, I have the ideal life."
How long will that life involve coaching Williams not even he knows.
"One day she will wake up and she will tell me 'I don't feel like going to practice tomorrow.' She'll say the same the day after, the week after, and then she'll tell me it's game over, but when that is going to be I have no idea," he says.
What Mouratoglu is sure of, however, is that Williams will be aiming to win grand slams when she returns to the sport.
"The goal is not to beat this record, or that record, the real goal is to set up Serena's record. Is it 23? 24? 25? 30? I don't know, but let's go, let's find out.
"Why would there be a limit? If, as a coach, I set limits for Serena I'm really a terrible coach because Serena doesn't have limits."
'McEnroe didn't mean to hurt Serena'
She might not have played since the Australian Open final, but Williams is still making headlines.
First her pregnancy caused waves, then she appeared naked on the front cover of Vanity Fair, and recently she's been involved in a spat with compatriot John McEnroe.
The three-time Wimbledon champion said Williams would be "about 700 in the world" were she on the men's tour. While others debated the matter, Williams responded on social media, asking the former world No.1 to respect her privacy.
There has not been an interview where Mouratoglou has not been asked about the outspoken New Yorker's comments.
"He didn't mean to hurt her, of course, but it's hurtful and I understand her reaction," he says. "It's basically saying women's tennis is not good. But one comment does not make a sport sexist."
At pain to emphasize he is not specifically referring to McEnroe, Mouratoglou, who has also coached on the men's tour, criticizes those who habitually compare men's and women's tennis.
"It's true women don't run as fast, they don't jump as high but because of that does it make their tour not interesting? I don't think so, it's just another tour," he says.
"You have to see women's and men's tennis as different sports because women have different qualities. I don't understand why people compare, it doesn't make sense to me.
"The message is -- it's not as good as men. OK, if you don't like it, don't watch it. You don't have to make a comment. Let the people who like it, like it."