Another highlight of the opening day at the Hangzhou Olympic and International Expo Centre was the short course championship record set by Danas Rapsys of Lithuania in the men’s 400m freestyle final, as he clocked 3min 34.01sec.

There were also gold medals for Australia’s Ariarne Titmus in the women’s 200m freestyle, Hungary’s three-time Olympic champion Katinka Hosszú in the women’s 400m individual medley and home swimmer Wang Shun in the men’s 200m individual medley.

The United States also claimed victory in the women’s 4x100m relay.

Seto set a world record of 1:48.24 as he was chased home by the man whose record he broke – South Africa’s multiple world and Olympic champion Chad Le Clos.

The latter athlete - who had publicly joined other swimmers such as Hosszú and Britain’s Olympic 100m breaststroke champion Adam Peaty on the eve of the Championships by voicing criticism of FINA’s refusal to sanction the lucrative new events being proposed by the International Swimming League - also broke his 2013 mark of 1:48.56, but 1:48.32 was only enough for silver.

Le Clos, who had been seeking a fourth consecutive title, led after 50 metres, but could not deal with the final surge produced by the man who took silver in this race four years ago in Doha.

Bronze went to China’s Li Zhuhao in 1:50.39.

"I am so happy and I am so surprised that I set a world record,” said Seto.

“I wake up every day wanting to set a new record and I finally did it - it is amazing.

“These Championships are related to the next two major milestones, South Korea next year, for the FINA World Championships, and the Tokyo 2020 Olympic Games for which I have the same goals, winning the gold in the 200 and 400 individual medley.

The US men’s freestyle relay team lowered the record to 3min 03.03sec

Rapsys swam a solitary effort to beat the best previous 400m freestyle championship mark of 3:34.32 established by Hungary’s Peter Bernek in Doha four years ago.

He thus became only the second men’s gold medallist for Lithuania in the history of the Championships, after the triumph of his compatriot Simonas Bilis in the 100m freestyle two years ago in Windsor, Canada.

Henrik Christiansen of Norway won silver in 3:36.64, with bronze going to Gabriele Detti of Italy in 3:37.54.

In the women’s 200m freestyle, Titmus won in 1:51.36, with Mallory Cornerford of the United States second in 1:51.81 and Femke Heemskerk of The Netherlands claiming bronze in 1:52.36.

In the women’s 400m individual medley, Hosszú - one of three swimmers who have taken out a US lawsuit against FINA over their refusal to sanction the ISL’s proposed debut event in Turin later this month - lived up to her position as overwhelming favourite.

After being on world record pace for half of the race, she dipped a little on the breaststroke leg but still came home a clear winner in 4:21.40.

Silver went to Melanie Margalis of the United States, in a distant 4:25.84, while the bronze was earned by France’s Fantine Lesaffre in 4:27.31.

The crowd’s high expectations in the men’s 200m individual medley were borne out as Wang Shun won in 1:51.01.

Josh Prenot of the United States took the silver in 1:52.69, while the bronze went to Hiromasa Fujimori of Japan in 1:52.73.

The United States got two relay gold medals at the end of the evening session, firstly winning the women’s 4x100m free relay in 3:27.78, and then, even more impressively, the men’s 4x100m free in a world record of 3:03.03.

That mark – jointly set by Caeleb Dressel, Blake Pieroni, Michael Chadwick and Ryan Held - improved the previous best performance of 3:03.30 set by the United States in December 2009.

In the women’s relay silver went to The Netherlands in 3:28.02, with China taking bronze in 3:30.92.

In the men’s relay, Russia were second in 3:03.11 and Brazil third in 3:05.15.