She grew up rooting for Serena Williams, even did a report on her way back in third grade. Her dream was to play her idol at the U.S. Open.
So when she had actually done it, beating Williams 6-2, 6-4 on Saturday night to become the first Grand Slam singles champion from Japan, why was it so difficult?
"Because I know that, like, she really wanted to have the 24th Grand Slam, right?" Osaka said. "Everyone knows this. It's on the commercials, it's everywhere.
"When I step onto the court, I feel like a different person, right? I'm not a Serena fan. I'm just a tennis player playing another tennis player. But then when I hugged her at the net ... I felt like a little kid again."
Osaka teared up as she was finishing her answer, still overwhelmed as she juggled the idea of her winning and Williams losing.
Though her nerves on the tennis court don't show it, it was a reminder of just how youthful the 20-year-old Osaka is. Not since Maria Sharapova was 19 in 2006 has the U.S. Open had a younger women's champion.
The way Williams lost, of course, was what stood out most in the match. The arguments with chair umpire Carlos Ramos and the three code violations — one that gave Osaka a game for a 5-3 lead in the second set when Williams was trying to rally — will be what was most remembered.
But not for Osaka, who claimed to not even hear the interactions between Williams and Ramos. What will stay with her is the hug at the net afterward, and Williams' kind words during the trophy presentation, when she asked a booing crowd to focus its intention on Osaka's moment.
"So for me, I'm always going to remember the Serena that I love," Osaka said. "It doesn't change anything for me. She was really nice to me, like, at the net and on the podium. I don't really see what would change."
Osaka was nervous Saturday, making a few phone calls to her sister in Paris to calm her down. Even during the match, whenever she was faced with a tough spot, she kept telling herself to try to do what Williams would do.
Williams was certainly impressed.
"She was so focused," the 36-year-old Williams said. "I think, you know, whenever I had a break point, she came up with some great serve. Honestly, there's a lot I can learn from her from this match. I hope to learn a lot from that."
It was that way throughout the tournament for Osaka, who won the second title of her career. She was mostly dominant, dropping only one set in her seven matches, and she saved 5 of 6 break points against Williams after erasing all 13 in the semifinals against Madison Keys.
That's the kind of toughness Williams has so often shown in winning 23 Grand Slam singles titles, one shy of the record. It's one of the things Osaka always admired about Williams, made her choose her as the topic of that report years ago.
"I colored it and everything," Osaka said. "I said, 'I want to be like her.'"
On Saturday, she was better.