One afternoon, as her older brother was practicing on the tennis court, Efremova picked up one of his rackets - as she often did - and began hitting a basket of balls.
"She started throwing the balls to herself," Julia Efremova, a former professional tennis player, told CNN.
"I looked at it and I was surprised because all the balls flew over the net and she made the moves perfectly. I told myself it was time to work with her because she had so much passion and she wanted have it.
"It was actually not my choice. She started her career like that."
Now 12 years old, Efremova is considered one of the smartest young talents in tennis and has already become something of a star, as she has gathered more than 35,000 followers on Instagram and secured sponsorship deals with Nike and Yonex.
She currently coaches at the prestigious Mouratoglou Tennis Academy in France, founded in 1996 by Serena Williams current coach Patrick Mouratoglou.
The academy holds selection weeks throughout the year where potential young stars come from all over the world to showcase their talents and try to get a spot.
"They come to be tested on the physical, tennis, of course, the mental side," Mouratoglou explains. "We see them practice. We also see them compete, and then we make a decision whether we want to help them or not.
"We can not help everyone, but we do the best we can to help those we believe have the best potential, and our role, of course, is to find a way to help them develop their potential and be the best. may be in the future to do so. "
'An incredible athlete'
Each player admitted to the academy has access to a coach, a fitness coach and medical team that gives them an elite support system from as young as nine years.
They are also provided with detailed debriefings of tournaments and competitive matches, with Mouratoglou saying that "competition stress" is the best way to see which aspects of their game need to be developed the most.
Individuals like Coco Gauff and Stefanos Tsitsipas are among the current rising stars who have spent a significant portion of their formative years at the prestigious academy.
Efremova was nine when she arrived at the academy from Russia with her mother, who also serves as her coach, and Mouratoglou says he immediately saw that her potential was "great".
"Ksenia has incredible potential, I think she has the full package," he says. "She's an incredible athlete. I mean, if you look at her social media, you'll see her. She can handle the full splits, she can dance, she can do all sorts of things besides tennis.
"She moves extremely well. She's probably going to be tall because her mother is very tall. Her shots are amazing. Her technique is extremely clean. She can take the ball early. She's aggressive. She's a very good competitor. So if you looking at the whole package, it's great. "
Julia believes that her daughter, who plays several sports, has played a major role in making her such a well-rounded athlete at such a young age.
Not only has it given her a range of transferable physical skills - such as endurance and flexibility - that have improved her as a tennis player, but Julia also says it was crucial to ensure that Efremova did not get bored playing too much tennis too soon. .
In fact, Julia says that her daughter's main sport growing up was gymnastics, where she often trained three hours a day compared to just one hour of tennis.
"I want to help parents all over the world who have a dream of building a professional tennis player from their children, because when they are young and of that age and they have this fire in their eyes, you can not kill it with hours of work. on the tennis court, ”she explains.
"For example, Ksenia played [tennis] only three times a week when she was little and I did not force her. I forced her in the other ways so she had dance lessons, swimming lessons, she had English lessons, she also had break dance lessons. She was everywhere. "
'I want to be a legend'
Few children will experience the kind of pressure and expectations that Efremova has already dealt with for years, but Mouratoglou says her situation can be compared to Gauffs as she rose in ranks.
Making mistakes is an important part of the process of learning to deal with that pressure, Mouratoglou explains, "because mistakes are understanding."
At the academy, he says, the children are taught how to express the pressure and the nerves they feel before a fight where it has affected them negatively.
"I mean, failing is not a good thing," he says. "Of course our job is to make them successful, but we know that on the road to success they will also be some failures - and there must be. What is important is that those mistakes are always used on a way to learn and get better.
"So when they fail because of the pressure, they have to know exactly how they felt before the match. They have to realize that they had extra pressure that day that they were unable to handle, and it has to be a constant feedback.
"They want to know that the next time they feel the extra pressure, they have to explain it. They have to tell the coach, 'I'm not feeling well today. I feel that pressure today. I feel nervous.' "First of all, they have to recognize it. Second, you have to express it, and when they do, we can help them work on it."
Of course, some children will be able to handle the pressure and nerves better than others. Mouratoglou says Efremova is still learning but has already shown remarkable improvements during her time at the academy.
While much of the pressure comes from the expectations of "everyone in the entire tennis industry," Mouratoglou explains that the incredibly high standards Efremova has for herself means she puts more pressure on herself than anyone else.
"She always expects to win," he says. "There is, for her, no choice but to win trophies."
Julia Efremova says she sees that determination in her daughter every day during training and believes "from the bottom of my heart" that "one day she will be the best" tennis player in the world.
"I know her personality. I know who she is. I know how hard she works. I know how [badly] she wants it. I know how she thinks, ”she says.
"First of all, she believes in herself. She has no doubt in her mind. She has no doubt in her heart that she wants it. So from the bottom of my heart, I know she's going to be the best. .
"Sometimes when I get angry and did not like anything during training, I ask her, 'What do you want from tennis?' And she tells me, 'I want to be a legend.' For her, it's not just [enough] to become number one in the world. "
To overcome tragedy
There is perhaps no greater testimony to Efremova's remarkable calm than her recent victory in the tournament, Tennis Europe Junior in Sweden.
Her father, former amateur Alexey Efremov, had been battling lymphoma for more than two years. During the tournament, Efremova's mother received the news that her husband would soon lose that match.
Julia says she had to make the difficult decision of whether to tell Efremova during the tournament or wait until it was over.
"It was very hard, but Ksenia was in the tournament and I had to tell her that," she says. "Of course she cried. She was shocked. She asked me, 'Maybe you can wake him up.'
"I said, 'No, Ksenia. It's impossible, he's already in heaven.' I asked her if she might stop, maybe we'll stop the tournament and she's come back.
"She said, 'No, I'm going to play this tournament to the end."
The final took place on Friday, December 3, just six days after Alexey passed away. Efremova won the final and dedicated the title to her father.
"In memory of my father, who died during this tournament in Sweden," Efremova wrote on her Instagram account, which is run by her mother. "1st place. You will always live in my heart as the strongest person in the universe. I will do anything to make your dreams come true and I know you will see it from above."
Julia says that Efremova's decision to continue with the tournament did not come as a big surprise, and explains that that kind of resilience is something she has inherited from her father. She knows it was a decision Alexey would have approved.
"Her father, he loved her so much," Julia says. "She has his spirit. He is a strong, very strong person, and she got that spirit from him."