She made her debut the same year the event was introduced at Beijing 2008. She was only 16 years old but produced a superb performance to place fifth.

She leads a field of 25 in an event which is timed to begin at 6.30am local time to counter the heat.

The temperature at the start of the race is expected to be 27 degrees and rising with humidity likely to reach 87 per cent.

The course has been set out at Odaiba Marine Park, the same venue which hosted triathlon earlier in the Games.

Rio 2016 champion Sharon Van Rouwendaal from the Netherlands is back to defend her title. She comes to Tokyo as European champion but has already competed here in the 200 metres breaststroke where she finished 16th.

Other leading European contenders include 2020 European silver medallist Anna Olasz of Hungary.

Xin Xin of China is also set to start. She finished just outside the medals in fourth at Rio 2016 but won 2019 World Championship gold in Gwangju and could become the first swimmer from Asia to win Olympic gold in marathon swimming.

American Haley Anderson finished only one second behind Xin in that 2019 final. She is competing in her third Olympics after winning silver at London 2012.

Rio 2016 silver medallist Rachele Bruni of Italy also returns. Since then she has won bronze at the Worlds in 2019.

The Japanese challenge is carried by Yumi Kida who competes in her third Olympics. She finished 12th in both London and Rio.