Eventually he became too strong for his grandfather, so he started taking formal lessons at a neighborhood shogi school.

[2] Fujii's intense focus surprised his mother and his competitive spirit often led to crying outbursts when he lost.

[2][3] Fujii was seven years old when met his future professional shogi mentor Masataka Sugimoto for the first time.

Sugimoto was amazed by the "marvelous perspective" and "insight" Fujii showed at such a young age and says that he is "the personification of fighting spirit" who "takes the frustration of defeat and directs it at the next match".

[2][4] Fujii officially entered the shogi professional apprentice school as a 10-year-old under the sponsorship of Sugimoto with the rank of 6-kyū in September 2012.

[10][11] On April 4, 2017, Fujii defeated Hiroshi Kobayashi to win his 11th straight game since turning professional and break the previous record of 10 jointly held by Yoshiyuki Matsumoto and Masakazu Kondō.

[12][13] Fujii won his 13th game in a row when he defeated Shōta Chida in the round one of the 67th NHK TV Shogi Tournament on April 17, 2017.

[15] Fujii became the fastest professional to reach 50 wins in official games on November 21, 2017, when he defeated Shingo Hirafuji.

His promotion to professional status and his subsequent success was viewed as helping the Japan Shogi Association recover from the scandal of the 29th Ryūō challenger controversy; a scandal which had ultimately led to the resignation of the JSA's president in January 2017, the removal of several directors in February 2017, and a general loss of public confidence.

[29][30] Fujii's streak not only helped the JSA recover from the aforementioned scandal, but also provided an economic boost as well.

[34] In December 2017, the JSA estimated that the over all impact of Fujii's streak on its advertizing revenue had been roughly 18.5 billion yen.

[35] Fujii's first official game against a reigning major title holder came on January 14, 2018, when he defeated Meijin Amahiko Satō in the quarterfinals of the 11th Asahi Cup Open [ja].

[37] Fujii became the youngest player to win a professional shogi tournament on February 17, 2018, when he defeated Akihito Hirose in the final of the 11th Asahi Cup Open.

Fujii advanced to the final by defeating the reigning Ryūō and Kisei title holder Yoshiharu Habu in the semi-finals held earlier that same day.

[38][39][40] On March 15, 2018, Fujii defeated Tatsuya Sanmaidō to finish Class 2 play undefeated at 10 wins and 0 losses.

The victory was Fujii's 15th in a row, which made him the holder of the two longest consecutive winning streaks for the 2017 shogi season.

[48][49] In October 2018, Fujii won his second shogi championship when he defeated apprentice professional Wakamu Deguchi 3-dan 2 games to 0 to win the 49th Shinjin-Ō [ja] tournament.

[52] Fujii, however, was defeated by Seiya Kondō in his next league game on February 5, 2019, to end the winning streak.

[54] In March 2020, Fujii became the first shogi professional to achieve a .800 winning percentage or greater for three consecutive years when he defeated Akira Inaba in challenger league play for the 61st Ōi tournament.

Fujii's victory not only allowed him to become the challenger for a major title for the first time, it also made him the youngest person ever (at the age 17 years, 10 months and 20 days) to challenge for a major title, breaking the record set 31 years earlier by Nobuyuki Yashiki by four days.

[75] In October – December 2022, Fujii defeated Akihito Hirose 4 games to 2 to successfully defend his Ryūō title.

[78][j] On February 23, 2023, he won the 16th Asahi Cup Open by defeating Toyoshima and Watanabe, in the semi-finals and finals respectively.

[82][k] On the same day, the final of the 72nd NHK Cup was broadcast[l] and Fujii defeated Yuki Sasaki 8-dan to win the tournament for the first time.

Fujii's victory also gave made him the youngest person at 21 years and 2 month to win the Oza title.

Fujii is the youngest player to win those four major awards in the same year and only the third shogi professional to accomplish the feat since 1967.

[131] Fujii was the only participant, which included both amateur and professional shogi players, to finish with a perfect score of 100 points.

[135] Fujii has been featured in several programs broadcast on the JSA's shogi channel of the Internet television station AbemaTV.

The games were unofficial, which meant the results did not affect his official win–loss record, and his opponents were (in order) Yasuhiro Masuda, Takuya Nagase, Shintarō Saitō, Taichi Nakamura, Kōichi Fukaura, Yasumitsu Satō and Yoshiharu Habu.

Fujii said that he gave becoming a full-time shogi professional serious consideration but stated "I'd like to continue to progress and make every experience a positive one."

Fujii's decision was widely anticipated in Japan and was viewed favorably by fellow shogi professionals and some education critics.