In pre-season, Anzai visited English Premier League club West Ham as part of a week long training camp, where he met the likes of Andy Carroll, Alex Song and Mark Noble.
Playing alongside such players, Anzai said that he felt the biggest difference was the physicality of the game in England and started more strength training on his return to Japan.
[14] Anzai continued to impress and was ever-present throughout, and by the end of the 2017 season was playing under his third manager at Verdy, following the dismissals of Yasutoshi Miura and Koichi Togashi.
After 12 years at Tokyo Verdy, in December 2017 it was announced that Anzai would be joining J1 League club Kashima Antlers, wearing the number 32 shirt.
[16] Although disappointed not to help promote his boyhood team Verdy to the top division, he would get a chance to play alongside Atsuto Uchida, a right-back and fellow countryman that he had long admired.
[22] Anzai played a total of 28 league games and scored three goals in his first season, in spite of a knee injury suffered in March that kept him sidelined for a month.
[23] In November 2018, Kashima won their first ever AFC Champions League title after a 2–0 aggregate victory over Iranian club Persepolis, with Anzai coming on as a substitute in both of the finals games.
Anzai played in most of the Champions League games, making 10 appearances in total, but was increasingly used as a wide midfielder rather than a full-back, including in the finals.
As reward for Kashima winning the Champions League, Anzai went on to represent them at the Club World Cup, making three appearances in the competition including coming on as substitute in their 3–1 defeat to Real Madrid in the semi-finals.
[29] Mid-way through the Japanese league season in July 2019, it was announced that Anzai would be transferring to Portuguese Primeira Liga side Portimonense, joining compatriot Shūichi Gonda in their squad.
[37] Once the league season had restarted after the break between March and June due to the COVID-19 pandemic, Anzai was used more as a substitute as Emmanuel Hackman appeared to be the new manager's first choice right-back.
[45] After a solid start to the 2019 season for Kashima Antlers, Anzai was called up to represent the Japan national team by manager Hajime Moriyasu, in the absence of regular full-backs Yuto Nagatomo and Hiroki Sakai.
[48] Anzai said that he perhaps didn't play to his best in the game against Bolivia, but the experience of training for 10 days with the national team and a taste of international football was exciting and would provide room for growth.
[49] Anzai was then left out of the squad that competed in the 2019 Copa América in June 2019, with Daiki Sugioka and Tomoki Iwata taking the starting full-back positions.
In October 2019, Anzai made his first competitive international appearance, coming on as a second-half substitute for Hiroki Sakai in a 6–0 win over Mongolia in a 2022 World Cup Qualifying game.