Despite the absence of many of the sport's biggest names, a world record - and a prestigious one at that - has been broken on the opening day of swimming at the world aquatics championships.

Teenager Pan Zhanle swam the fastest 100 metres in history as he led off China's gold medal-winning 4x100 freestyle relay team on Sunday (Monday AEDT).

His time of 46.80 seconds - with a split time of 22.26 - took six-hundredths off Romanian David Popovici's world record from 2022 and established the 19-year-old as one to beat in swimming's marquee race at this year's Paris Olympics.

"Actually, I wasn't feeling in great form," Pan said after the race.

"I would've been happy with just getting on the podium.

"This morning (in heats), the US, Great Britain and Italy were all quicker than us, and I just wanted to focus on doing my job well.

"I told myself, 'Just swim hard'. I was shocked when I saw the time, I wasn't expecting to break the record now - I wanted to leave it for the Paris 2024 Olympic Games."

China won the relay in 3min 11.08sec, Italy finished a second behind, with the United States rounding off the podium in 3:12.29.

Erika Fairweather became New Zealand's first ever gold medalist at a world aquatics championships by winning the women's 400 freestyle.

Fairweather, who claimed bronze behind Australia's Ariarne Titmus and American Katie Ledecky at last year's swimming worlds, won in 3:59.44, 2.18sec ahead of Li Bingjie, of China.

Germany's Isabel Gose was third, 2.95 behind Fairweather, who also broke her national record on her way to victory.

With open water, artistic swimming and diving events all completed in Doha, the focus shifted to the Aspire Dome and eight days of racing in the pool.

Titmus and Ledecky were just two of the sport's biggest names skipping the worlds, with the Olympics starting in Paris in just over five months.

It is the first time in history a long-course world championships has been held in the same year as an Olympics.

The men's 400m free final was also without a defending champion but for a different reason as Ahmed Hafnaoui was surprisingly eliminated in the heats earlier in the day, finishing two seconds off the pace.

Kim Woo-min won in a personal best of 3:42.71 to claim South Korea's first medal in the discipline since 2011, edging out 2022 champion Elijah Winnington of Australia by 0.15 and Germany's Lukas Martens by 0.25.

In the 4x100m women's free relay, the Netherlands took the gold in 3:36.61, with Australia earning the silver (3:36.93) and Canada the bronze (3:37.95).

In men's water polo, Italy edged the United States 13-12 to reach the quarter-finals.

Croatia, France and Montenegro also advanced.