TOKYO — When the Ohno family watched Naomi Osaka beat Serena Williams at the United States Open early Sunday morning from Tokyo, there was no question in their minds that Ms. Osaka was a true Japanese champion.
“Her face looks Japanese,” said Ryutaro Ohno, 14, shortly after playing a few tennis matches with his younger brother and parents at a court near the base of Tokyo Tower.
His mother, Naoko, 49, showed a snapshot on her cellphone of her sons posing with Ms. Osaka, the daughter of a Haitian-American father and Japanese mother, when the tennis star played in the Pan Pacific Open in the Japanese capital last year.
“Her soul is Japanese,” Ms. Ohno said. “She doesn’t display her joy so excessively. Her playing style is aggressive, but she is always humble in interviews. I like that.”
In becoming the first Japanese-born tennis player to win a Grand Slam championship, Ms. Osaka, 20, is helping to challenge Japan’s longstanding sense of racial purity and cultural identity.
Her emergence comes at a time when Japan is also grappling with a declining population, a looming demographic crisis that has prompted the country to open its doors slightly to accommodate an increase in foreign residents and descendants of Japanese immigrants who want to return to Japan.
Yet even as a new generation starts to embrace a broader sense of what it means to be Japanese, a conservative strain in the country clings to a pure-blood definition of ethnicity. Still, the Japanese media warmly welcomed Ms. Osaka’s victory as the country’s own.
“The first Japanese achievement,” read a headline on a special edition of the Sankei Shimbun on Sunday, over a large picture of Ms. Osaka, who moved to the United States when she was 3, kissing the trophy.
On Twitter, Prime Minister Shinzo Abe congratulated Ms. Osaka. “Thank you for giving energy and inspiration to all of Japan in this troubled time,” he wrote, referring to two natural disasters of the past week: a typhoon that ripped through Western Japan and an earthquake that rocked the northern island of Hokkaido.
Many fans woke near dawn to watch Ms. Osaka in the final. At the headquarters of Nissin, an instant-noodle company that is one of Ms. Osaka’s corporate sponsors, 150 employees gathered to watch. Nissin’s chief executive, Koki Ando, told the Nikkei newspaper that her victory was “hanpa nai ne,” Japanese slang for “awesome” or “extraordinary.”
Kei Nishikori, Japan’s biggest male tennis star, used emojis to congratulate Ms. Osaka on Twitter, interspersing a series of thumbs up, trophies, flexed arms and fist bumps with several Japanese flags. Mr. Nishikori reached the semifinals of the tournament, but lost to Novak Djokovic.
The celebration of Ms. Osaka struck some in Japan as hypocritical. Many biracial people — known as “hafu” in Japanese, a term that comes from the English word “half” — say that they are not truly accepted.
“I feel sick to see people who say that Naomi Osaka is a Japanese or the pride of Japan,” a user with the handle @phie_hardison wrote on Twitter. “You can’t embrace a ‘hafu’ as Japanese only in such times. They are usually discriminated against, aren’t they?” The post had been shared more than 3,600 times as of 10 p.m. on Sunday in Japan.
Three years ago, when Ariana Miyamoto, a half-black, half-Japanese woman, was crowned Miss Universe Japan, the judges received some criticism online from people who said she did not look sufficiently Japanese.
Baye McNeil, an African-American columnist who writes for the Japan Times about the black experience in Japan and who has lived in the country for 14 years, said the celebration of Ms. Osaka presented a racially progressive view that did not align with a messier reality.
“This country prides itself on being homogeneous,” Mr. McNeil said. He said that to have a woman of mixed cultural heritage rise in the spotlight placed many Japanese “in an awkward position of sending a message to the world that they’re in a place that everyone knows they’re not.”
But Japan, however slowly, may be changing. The year after Ms. Miyamoto won the beauty contest, another mixed-race woman, Priyanka Yoshikawa, took the crown.
Megumi Nishikura, co-director of the documentary “Hafu: The Mixed-Race Experience in Japan,” said, “Anybody who is able to represent Japan in a public way who is ‘hafu’ will open Japanese minds and hearts to being more accepting.”
“We live in a world where people have a limited view on nationality and race and ethnicity and say that you can only be one, you can’t be more,” added Ms. Nishikura, who was raised in Japan by a Japanese father and Irish-American mother and who now lives in New York. “I think Naomi Osaka really presents a very interesting challenge for people who are still attached to these antiquated ideas that you can only be one.”
In Tokyo on Sunday, there were signs those antiquated ideas might be shifting. “I think the definition of what is Japanese is becoming vague,” said Masako Mikami, 52, who works at a gaming software company in the city. “I think Japanese society is changing to become more generous. Naomi Osaka is one of the next generation of Japanese people.”
Shinji Ichinose, 36, who watched the final, said that he had once thought it would be impossible for a Japanese woman to win a Grand Slam title.
Mr. Ichinose, who was getting ready to play tennis at dusk on Sunday, said he had the perception that Americans or Europeans put more emphasis on nationality than a person’s appearance. “So I think Japan may change to be more like that in a future era,” he said.
Ms. Osaka has worked to cultivate her Japanese identity on the circuit. In interviews with Japanese outlets, she answers questions in her imperfect Japanese, and she has talked about her love of manga and green tea.
After her victory, Ms. Osaka demonstrated a characteristically Japanese trait when she apologized for her win. “I’m sorry it had to end this way,” Ms. Osaka said, acknowledging the contentious decisions against Ms. Williams by the umpire. As she thanked the fans, she dipped her head in a bow, also a common practice in Japan.
In an interview with the Japanese public broadcaster, NHK, Ms. Osaka answered some questions in Japanese and others in English. In Japanese, she said that she was “a little sad and a little happy” to have defeated Ms. Williams. But when an interviewer from the Tokyo Broadcasting System asked how she felt about being Japan’s first Grand Slam champion, she responded in English. “I’m very honored. … I don’t know how to say that in Japanese,” said Ms. Osaka from a studio in New York.
Supporters in Japan said they were disappointed that the final had ended in such a storm. “But she really won with her ability,” said Kanako Ozawa, 32, a real-estate agent who was hitting balls at a tennis court in central Tokyo on Sunday.
Darryl Wharton-Rigby, an African-American filmmaker from Baltimore who is married to a Japanese woman and has three children, said that Ms. Osaka’s win had special resonance and said he felt optimistic about Japan becoming more racially progressive.
“I actually don’t like using the word ‘hafu,’” he said. “I prefer biracial or bicultural because when people say half, it sometimes feels ‘less than.’ You’re only half Japanese, which means you’re not fully Japanese. But I think that that’s a thing that could potentially change with every victory or every person who comes out and says, ‘No, I’m Japanese, even though I have a parent who isn’t from Japan.’”
Follow Motoko Rich on Twitter: @MotokoRich.
Makiko Inoue and Hisako Ueno contributed reporting.