Ferry Weertman of The Netherlands is back to defend the title he won after a furious finish to the race at Rio 2016, when two photo finishes were needed to establish the three medallists.

Marc Antoine Olivier of France claimed bronze at Rio and is the only man to have won a medal at both World Championships since then.

Oussama Mellouli of Tunisia, the champion at London 2012, remains the only swimmer to win Olympic gold in both the pool and open water.

Florian Wellbrock of Germany won 1500 metres freestyle bronze in the pool four days ago.

He had been the first man to qualify for the Olympics when he won gold at the 2019 International Swimming Federation World Championships.

Gregorio Paltrinieri can become the first Italian man to win an Olympic medal in marathon swimming. 

He could also win a medal in both pool and open water here after an 800m freestyle silver last week.  

In 2016 he won 1500m freestyle gold in Rio.

Also in contention in a field of 26 is Hungary’s Kristof Rasovszky who won the FINA Marathon Swim World Series in 2019.

Japan’s Taishin Minamide won double gold at the Asian Championships in 2018 and carries the hopes of the host nation.

The race starts at 6.30am local time so most swimmers are expecting to get up at 3am.

The temperature at the start of the race is expected to have already reached 26 degrees, rising to 31 degrees by the end of the race.