Japan sparked in joy after Naomi Osaka won back-to-back Grand Slam titles at the Australian Open on Saturday, with fans taking pride in the feat that guaranteed she will become the country's first world No. 1 in tennis.

In the namesake city of Osaka in western Japan, where she was born, around 30 fans waving Japanese flags erupted in joy after watching the grueling final match on an installed TV screen at a tennis center where Osaka practiced in her childhood.

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"I'm deeply moved. I can only say she's fantastic," said 58-year-old Hiroshi Kawai, who visited the Utsubo Tennis Center with his 8- and 10-year-old children.

"I think she was able to win because her serves were on target," said 9-year-old Chihiro Matoba, after Osaka beat the Czech Republic's Petra Kvitova.

In Nemuro, Hokkaido, in northern Japan, Osaka's 74-year-old grandfather Tetsuo Osaka told reporters, "I'm proud of my granddaughter. I want to tell her congratulations."

He said he did not talk with Osaka before the match as he did not want to put pressure on her. "The game had many nail-biting moments and I could not move away from TV," he said.

Of Osaka he said, "She was pampered and has some weakness. But she fought well on a big stage even as she was sulking."

(Tetsuo Osaka)

Tennis star Kei Nishikori did not react to his compatriot's win with words but with dozens of emojis showing thumbs up, flexed biceps, fists, trophies and Japanese flags in a Twitter post.

Staff at a Tokyo showroom of Yonex Co., a sporting goods maker that as a sponsor provides tennis rackets to Osaka, also closely watched the match. A prepared panel showing Osaka's photo and the word "CHAMPION" was put up as soon as she clinched the win.

"I never thought a Japanese would win two Grand Slam tournaments in a row. I want to say thank you to Osaka," said teary eyed Masahisa Nomura, one of the staffers.

Among some 100 supporters who gathered to cheer Osaka at a tennis school in Niiza, Saitama Prefecture, near Tokyo, 12-year-old Reira Nasu was basked in excitement.

Osaka "doesn't make mistakes even if she is aggressive on offense. And she can also rebuild her game even when she is under pressure. I want to learn from her mental strength," she said.

Prime Minister Shinzo Abe also took to social media to praise Osaka's accomplishment. "It was an impressive win following a very close match!" his message on Instagram read. "I feel very proud of the birth of the new world queen. I hope for her further success."

Former Japanese professional tennis player Ai Sugiyama said Osaka has grown mentally.

"Even when she wasn't doing well, she used her time during points effectively and switched her mindset positively," Sugiyama said.

"The victory is a result of her growth in two stages -- mentally and in terms of skills," she said. "She did not lose to anybody even though it's very difficult for players to be prepared for the Australian Open, the first major tournament of the season."