Supporters in a packed Tokyo bar had a rollercoaster ride watching the action from the Australian Open final as their heroine Osaka contrived to give up three Championship points in the second set.
But as Osaka claimed victory and sunk to her knees in triumph and disbelief, fans let out the pent-up emotion following a thrilling see-saw battle with eighth seed Petra Kvitova from the Czech Republic.
They punched the air, clapped hands and gave each other hugs and high fives as shouts of joy rang around the B ONE bar in upmarket Ginza, replacing earlier chants of "Naomi! Naomi! Naomi!"
Prime Minister Shinzo Abe led his nation's congratulations, hailing an "impressive victory in a very tight game".
"I'm so proud of the birth of the new world queen," tweeted Abe.
In securing victory in Melbourne after her stunning win over Serena Williams in the US Open last year, Osaka also clinched the world number-one spot, the first Asian to do so.
Her US Open victory was the first by a Japanese player and she is now the youngest woman to win back-to-back majors since Martina Hingis in 1998 and the youngest number one since Caroline Wozniacki in 2010.
With her gutsy performance in Melbourne backing up her breakthrough in New York, Osaka confirmed herself as the new star in the women's game and fans were expecting her to kick on from here.
"She can win much, much more! I want her to win on grass... I want her to win at Wimbledon!" Masako Takeda said at the bar as she wiped away tears of joy.
Another fan, Aki Tani, praised Osaka for being "humble but playing boldly" on court.
"I'm so happy that we can tell she has grown so much over the past two years!" she said.
Nozomi Hirabayashi noted it was "unthinkable before" that a Japanese player could become the world's best.
"I'm so proud," he said. "I want her to win the next Grand Slam as she is very mentally strong."
News flashes screamed across Japanese networks after the nail-biting match.
The Sankei newspaper tweeted: "She scored a remarkable win that makes us see 'the Era of Naomi' coming up."
Osaka has a Japanese mother, a Haitian father and was raised in the United States.
She has dual Japanese-American citizenship and often replies to questions from Japan's media in English, apologising for not knowing the appropriate word when she speaks Japanese.