Relegation returns to the Japanese top flight this season as the 2021 campaign kicks off on Friday with a clash between the competition's two most recent champions - Kawasaki Frontale and Ange Postecoglou's Yokohama F Marinos.

Japan's first division, known as J1, will feature 20 teams for the first time since the league's inaugural season in 1993 after relegation to the second tier was suspended last year as a result of the COVID-19 pandemic.

The impact of the health crisis on the 2020 schedule meant two teams - Tokushima Vortis and Avispa Fukuoka - were promoted but with none going down in the opposite direction four sides will be demoted at the end of the upcoming campaign.

Kawasaki Frontale won the trophy at a canter last year, finishing 18 points head of nearest rivals Gamba Osaka to claim their third title in four seasons. Cerezo Osaka and Nagoya Grampus filled out the top four spots to qualify for the Asian Champions League.

Frontale, coached by Toru Oniki and buoyed by victory over Gamba in the season-opening Super Cup on Saturday, will be favourites to win the title again and kick off their defence against 2019 champions Yokohama F Marinos.

Postecoglou's Yokohama struggled in 2020, finishing 36 points behind Frontale in ninth, and the Australian knows the new season will be just as challenging.

"Last year Kawasaki were outstanding and ran away with it, but there wasn't much between second place and the others," the Asian Cup-winning coach told Optus Sports.

"This is my fourth year and it is a very competitive, even league and with four going down every game with an edge to it.

"Kawasaki are a fantastic team; watching them in the Super Cup on the weekend, they've strengthened; Gamba have strengthened, Nagoya have bought some players to add to a strong squad - there are going to be some real key games and some tough challenges."

Former Barcelona midfielder Andres Iniesta remains one of the league's biggest draws, although the Spanish World Cup winner will be looking to inspire Vissel Kobe to a better showing than last year's 14th-place finish.