The shock move follows another disastrous result for Alesi, the son of ex-Formula 1 driver Jean, in last weekend's Sugo race as he spun on the opening lap and was collected by Yuhi Sekiguchi, eliminating both drivers.
Alesi had gone into the 2023 campaign, his second as a full-timer at TOM'S, under big pressure as the team elected to keep him in place after trialling two-time race winner Sasahara in last year's post-season Suzuka test.
TOM'S director Jun Yamada told Motorsport.com that commercial factors had been the main consideration in allowing Alesi to race on for another year despite the Frenchman scoring points only once in 2022.
After failing to finish either race in the Fuji opener, Alesi registered his first points of the season in April's Suzuka race in eighth, but was a disappointing 13th in the following race at Autopolis before his early Sugo DNF.
Alesi had told Motorsport.com after his collision with Sekiguchi that he had hoped to use this week's two-day test at Fuji as a chance for a reset.
However, on Tuesday, the bombshell news arrived that his seat will be taken over by Sasahara, who had taken an unofficial reserve position within the Toyota stable after losing out on a TOM'S drive this year.
Alesi will continue to race on for TOM'S in SUPER GT, where he shares the #37 Toyota GR Supra with Sasahara.
A brief statement from the team read: "After many discussions with Giuliano himself, the team has been deliberating how best to achieve the future growth and success of a driver who has been a member of the team since 2021.
"As a result of these deliberations, we believe that Giuliano forfeiting the final four races of the Super Formula season and focusing on SUPER GT, the other top category in Japanese racing, is the best solution for his future."
Alesi's lacklustre performances have been put into sharp focus by team-mate Ritomo Miyata, who currently leads the championship following his second victory of the year in last weekend's Sugo race.
Miyata now holds a 12-point lead in the championship race over Red Bull junior and Mugen driver Liam Lawson.
Otsu set for double B-Max duty at Fuji test
Honda driver Hiroki Otsu will get to drive a Super Formula car again when he joins B-Max Racing for this week's Fuji test.
Otsu will pull double duty in the two-day test, driving both of B-Max's SF23s alongside regular drivers Nobuharu Matsushita and Raoul Hyman.
Although he lost his Dandelion race seat to Kakunoshin Ota after the 2022 season, the 29-year-old made a one-off return at Autopolis last month, replacing an unwell Tomoki Nojiri at Mugen.
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