On a wet December morning in a South Florida weight room, the 21-year-old who stunned Serena Williams at the U.S. Open is hard at work preparing to show that the biggest moment of her life was more than a fluke. As an arrow flashes on an iPad in front of her, Naomi Osaka darts in the direction it signals, pauses, then pivots when it sends her the other way, without missing a step. Her coach, Sascha Bajin, joins the drill but leaps the wrong way and almost lands on Osaka’s ankle. Bajin feigns horror, prompting fellow pro tour player Monica Puig to suggest Osaka give her coach a hug. “She gives hugs like no other,” Bajin says, his sarcasm thicker than midsummer heat. “I only hug people I like,” Osaka parries.

The exchange would be unremarkable were it between almost anyone else. But Bajin’s playful banter is a key part of his strategy to break his young charge out of her shell. And for Osaka, a precocious talent in a global sport with the kind of multinational background that marketers dream about, doing so could mean the difference between a career like that of the idol she upset at the Open–or, well, a fluke. “It’s easier to take over the world,” Bajin says, “if you’re not so caved in.”

Many people’s introduction to Osaka came in September at the U.S. Open trophy presentation, when the surprise champion covered her eyes with her visor as boos rained from the crowd. “I didn’t want people to see me crying,” Osaka tells TIME, “because that’s pathetic.”

The moment should have been celebratory–a rising star assuming her place among champions after defeating the greatest of them all. Instead, it was painful. Thousands of fans, livid that umpire Carlos Ramos assessed Williams a code violation for verbal abuse that cost her a full game late in a Grand Slam final, filled Arthur Ashe Stadium with jeers. Rage pierced the still air, as if a wrestling heel were entering the ring and not a 20-year-old being honored for finishing a fairy tale.

Standing on the podium for the ceremony, tennis legend Chris Evert says she just wanted off. “I’ve never seen or felt anything like it,” she says. “The negativity, the anger.” From his seat, Bajin seethed: “I wanted to jump everybody in the crowd.”

At first, Osaka thought the boos were for her. She knew the crowd, and millions more watching on TV, desperately wanted Williams to win a record-tying 24th Grand Slam title after she nearly died after giving birth. When it was her turn to speak, Osaka apologized for doing her job and beating her opponent. And so it was that the woman who could be the heir to the Williams sisters met the world through a frowning face and lowered brim.

Three months later, Osaka is relaxing on the balcony of the Evert Tennis Academy in Boca Raton, Fla., where she trains. She doesn’t fault Williams for fighting with the umpire and upstaging her victory. “Serena is Serena,” Osaka says in her first extended interview since the match. “I didn’t experience her life. I can’t tell her what she’s supposed to do, because there are things that she’s gone through. I have nothing against her or anything. I actually still really love her.”

Osaka insists she’s come to terms with it all. She appreciates that Williams did eventually implore the crowd to stop jeering and applaud Osaka with a proper, if belated, ovation. In fact, Osaka insists she wouldn’t change anything about what happened. “In a perfect dream, things would be set exactly the way you would want them,” she says. “But I think it’s more interesting that in real life, things aren’t exactly the way you planned. And there are certain situations that you don’t expect, but they come to you, and I think those situations set up things for further ahead.”

The future actually came ahead of schedule for Osaka when she stormed through the field in New York, and she and her team are scrambling to capitalize. Born in Japan to a Haitian father and Japanese mother, Osaka grew up in the U.S. but competes for Japan. She has become a bankable celebrity in her native country and a source of inspiration to many multiracial people there. With the 2020 Summer Olympics in Tokyo, global companies are falling all over themselves to align their message with her 120 m.p.h. serves.

“If you’re talking about an international sporting event like the Olympics,” says Bob Dorfman, creative director of Baker Street Advertising in San Francisco and a veteran sports marketer, “she’s your international star you’re going to market it around. She’s got American appeal, Caribbean appeal, Japanese appeal. As nationalities continue to mix in this world, that makes her even more desirable.”

First, however, Osaka needs to keep winning. Her biggest test yet will come at the Australian Open, which begins on Jan. 14 in Melbourne. The great players have a way of finding another gear in the Grand Slam spotlight. Osaka has shown that she has the power game to beat the best. But can she do it when everyone is expecting her to–and millions of dollars are riding on it?

At the U.S. Open trophy ceremony, Osaka apologized to booing fans for beating her idol, Serena Williams Julian Finney—Getty Images

Osaka’s road to the top of tennis traces to 1999, when her father, Leonard Maxine Francois, watched a young Venus and Serena Williams playing in the French Open on TV. He heard the story of their hard-driving father, who groomed his daughters despite being a tennis novice, and figured he could do the same. “I always thought I could have been a great athlete if I had that support,” Francois says.

He had met Tamaki Osaka earlier in the decade, as a college student from New York studying in Sapporo. Over the objections of Tamaki’s father, who did not approve of the relationship, the couple married. They had two daughters, Mari and Naomi, born 18 months apart in Osaka. For practical purposes in a country that can be hard for outsiders to penetrate, the girls took their mother’s surname.

Inspired in part by the Williams sisters’ path, the family left Japan for the U.S. when Naomi was 3. They moved into the Long Island home of Leonard’s Haitian parents, eating beans and oxtail and hearing Creole around the house. Tamaki spoke both Japanese and English to the kids, and kept Japanese customs like Hinamatsuri, the March 3 celebration of girls’ health and happiness. The sisters went to public school, but their lives revolved around tennis. “It wasn’t really our choice,” says Mari, 22, now a professional player who competes in lower-level events.

Still, they liked the game enough to train for hours at public courts on Long Island. And for the girls, the Williams sisters became the models that Richard Williams was for their father. Naomi even did a third-grade report about Serena.

The family plan intensified in 2006 when they moved to Florida, the epicenter of American youth tennis. The kids were homeschooled online and dedicated even more time to honing their craft. “I’m the type of person when I want to do something,” Francois says, “I just go for it.”

The Osaka girls, like the Williams sisters before them, largely eschewed the junior tennis circuit, a cutthroat environment that burns out many promising teen players. Instead, they battled each other every day. “She was sort of the driving force,” Naomi says of her sister. “Because when we were little, I wasn’t really too good. I was just there. I didn’t really care. I was just playing because she was playing and I wanted to beat her.”

As Naomi started winning, it deepened her determination. “Once she puts her focus on something, she never strays from it,” Mari says of her younger sister. “It gets to the point where it’s almost ridiculous.” Mari’s favorite example is not on the court but rather her sister’s penchant for eliminating virtually all fat from her food, even if it takes 20 minutes to trim every piece of meat she eats. “What the hell?” says Mari. “How do you have the time and dedication? But she’s obsessed.”

Naomi was promising enough to turn pro in 2012, when she was 14. She climbed the rankings quickly: at the end of 2014, she stood at No. 250 in the world. Two years later she was ranked No. 40 after reaching her first WTA tournament final, and making the third round of all three Grand Slam tournaments she played. Osaka was named 2016 WTA Newcomer of the Year.

But there’s a chasm between the good players on tour and the great ones, who regularly contend for Grand Slams. Many close observers credit Osaka’s move into the latter group partly to the decision to work with Bajin at the end of 2017. A 34-year-old Serb born and raised in Munich, he spent eight years as Serena Williams’ hitting partner before coaching Osaka. “I saw tremendous improvement in mobility around the court,” says Evert, who analyzes the tour for ESPN. “The transformation, in a year, was unbelievable.”

Osaka plays a power game similar to her idol’s, relying on big serves and even bigger shots rather than defense and finesse. Bajin, who knows the style well from his time with Williams, helped Osaka refine her approach. “I see her hit balls late, and she just directs them down the line and they go like freaking rockets,” he says. “My heart freaking stops.”

In March, Osaka won the competitive Indian Wells tournament, and at the next event, in Miami, she crushed Williams in straight sets in the first round. Williams was in the early stages of her comeback, but the win confirmed that Osaka was someone to reckon with.

Osaka entered the U.S. Open on a three-match losing streak. But she says the losses eased her mind. “I sort of had this feeling of freedom,” she says. “At that point I felt the lowest I could be, so I honestly just wanted to recapture the fun feeling.”

After Osaka thumped Williams in the final, her life changed in an instant. The awkward tennis prodigy was now something of a celebrity, which has been an adjustment. In November the sisters attended a Drake concert in Miami, and Osaka froze after she realized people were shouting her name as she danced awkwardly. (She says “sitting still in my chair” is her go-to dance move.) Another whoa moment: while driving in Florida after the Open, she noticed a woman in front of her looking repeatedly into a side mirror. At a green light, the other car stood still. Osaka steered around and saw the woman’s mouth agape. “She was just looking at me,” Osaka says. “I thought it was because of my car. Then I realized I think it was because of me.”

Osaka received a hero’s welcome during a November trip to Haiti, and her fame in Japan is approaching pop-star status. When she visited Tokyo in September, Osaka had to sneak into her hotel through a side entrance. Paparazzi trailed her throughout the trip. One night, Osaka’s mom Tamaki was relaxing in her hotel room and decided to conduct a little test. She’d flip around the channels and see if she could finally avoid the image of her daughter on the screen. Her experiment failed.

Osaka, playing in an Australian Open warm-up tournament on Jan. 3, has one of the strongest serves in the game TPN/Getty Images

Osaka’s connection to Japan is both implicit and complicated. She was born there but has lived in the U.S. since she was 3. She is conversant in the language but typically responds to questions from Japanese reporters in English. Still, when the girls were junior players, their parents decided their daughters would represent Japan in international competitions, given the family’s cultural ties to the country. The decision has paid dividends. As the first woman from Japan to win a Grand Slam, Osaka is a pioneer. If she competed as an American, it wouldn’t be a milestone at all, and the battle for attention and endorsements would be more difficult.

Despite the affiliation, Osaka says she doesn’t feel more attached to one part of her identity than to any other. “I don’t really know what feeling Japanese or Haitian or American is supposed to feel like,” she says. “I just feel like me.”

Japan is one of the most homogenous places in the world. Around 98% of the population is ethnic Japanese, and being multiracial–or what’s known as hafu, or half–can be fraught. Carla Capers, an English teacher in Kobe whose parents are African American and Japanese, says co-workers often ask her if she can understand Japanese phrases. “I’m like, ‘I live here, I speak the language,'” says Capers. “People kind of dumb everything down. It gets really annoying.”

For those who see the possibility of a broader definition of what it means to be Japanese, Osaka has become a symbol. “It means a lot to me, it means a lot to my students who are mixed to see her on TV representing Japan, and seeing a resemblance,” says Harmony Egbe, a first-grade teacher in Okinawa whose mother is Japanese and father is Nigerian. “There’s an unspoken definition of what it means to be Japanese,” says Megumi Nishikura, co-director of the 2013 film Hafu: The Mixed-Race Experience in Japan. “Follow the customs, speak the language fully, look Japanese. She doesn’t click many of those boxes. That poses a challenge. People are having to redefine Japanese identity. She’s helping spread that conversation, which is remarkable.”

Japan’s leading companies have taken notice. The Citizen watch model that Osaka wore for the U.S. Open final almost sold out after her win. In the U.S., sales of the strings Osaka used on her Yonex racket rose 155% in the fourth quarter of 2018 compared with the previous quarter. Nissin, the instant-ramen giant, put her face on its cup of noodles. Among the other major deals announced since the Open: a sponsorship with Shiseido, the cosmetics company, and an agreement with automaker Nissan, which recently released a special-edition model to commemorate the partnership. A deal with an airline is likely to follow, as are those tied to major Olympics sponsors and an apparel company–her contract with Adidas conveniently expired at the end of 2018.

Osaka’s agent declined to reveal her endorsement income, but a person with knowledge of the market has estimated that she will go from earning about $2.5 million per year before the U.S. Open to taking in north of $15 million annually afterward.

Eight-figure investments come with thick strings attached. Osaka’s sponsors expect her to keep winning and to function as the public face of their brands. Osaka generally prefers to keep hidden. “Everyone around me has more confidence in me than I do in myself,” Osaka says. She’s given to self-deprecating comments like “I think everyone is cooler than me,” which come across as sincere rather than false modesty. And she excessively apologizes, for things large and small. Osaka said she was sorry for beating Williams, though no one deserved that victory more. And after one of our interviews, Osaka apologized for stepping over my computer bag, even though it was in her path.

Some of this comes from spending your childhood chasing tennis dreams rather than being social. “To go out of the way to make a friend, for example, you would have to say hi the morning, text them sometimes,” says sister Mari. “She doesn’t really put in the work for it.”

When asked her favorite moment of the post–U.S. Open victory tour, Osaka doesn’t mention going on Ellen or meeting LeBron James, one of her favorite athletes. Her pick: a trip to Universal Studios while in Singapore for the tour finals. “I got to skip the lines and stuff,” says Osaka. “So that was fun.”

Osaka is a star without the pretense, a multimillion-dollar corporate investment who still quotes Pokémon and predicts that fans should expect “just a whole bunch of awkwardness” from her off the court. Mari says she hasn’t noticed much of a change in her kid sister, aside from her more frequent shopping excursions online. “She’s going crazy,” says Mari. “Every day is like Christmas.”

If Osaka hasn’t changed, the expectations for her have. She’ll enter the Australian Open ranked fourth in the world and favored to make a deep run. But the field is loaded. The defending champion, Caroline Wozniacki, is ranked No. 3 in the world, while the two top-ranked players, Simona Halep and Angelique Kerber, each won a Grand Slam last year. Meanwhile, Serena Williams still looms. Williams reached the finals of the last two major tournaments. And the last time she played in Melbourne, in 2017, she defeated sister Venus in the final–while two months pregnant.

The end of the Williams era may not be here, but it is in sight. Osaka is wary of any “next Serena” label. She’s quieter than her idol, and she owns just one Grand Slam trophy to date. But she knows that it’s there for the taking. “You really never know what people can do,” Osaka says. “And how people can change. I don’t think there is ever going to be another Serena Williams. I think I’m going to be me. And I hope people are O.K. with that.”

This appears in the January 21, 2019 issue of TIME.

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Write to Sean Gregory at sean.gregory@time.com.