TOKYO - Eighteen-year-old Ahmed Hafnaoui of Tunisia stunned the pool when he swam an event-winning time of three minutes and 43.36 seconds in the men's 400m freestyle, as the first four swimming medals of the 2021 Olympic Games were awarded Sunday morning.

The 18-year-old youngster also ended the dominance of Australia, New Zealand and Asian nations in the men's 400m freestyle. A swimmer from outside Asia or Oceania has not won a gold medal in the event at the Olympic Games since Barcelona 1992.

Ahmed Hafnoui, Tunisia's second Olympic swim finalist ever, stepped on the podium in his shorts.

"I just can't accept that - it is too incredible," said Hafnaoui,

"Oussama is a legend. I wish I could become like him," he hailed his compatriot three-time Olympic medallist Oussama Mellouli.

Jack McLoughlin of Australia took the silver in 3:43.52, and American Kieran Smith claimed the bronze in 3:43.94. Top seed Germany's Henning Muhlleitner missed the podium, ranking fourth in 3:44.07.

In the women's 400m freestyle relay final, Australia grabbed a new world record in 3:29.69. The previous two records were also held by the Australians. Canada and America took the silver and bronze respectively. China broke Asian record in 3:34.76.

Yui Ohashi won the women's 400m individual medley gold for Japan in 4:32.08. American swimmer Emma Weyant and Hali Flickinger shared the podium.

Defending Olympic champion Katinka Hosszu of Hungary finished fifth. She has won three Olympic gold medals, including one from the women's 400m IM at Rio 2016.

In men's 400m IM, Rio Olympic runner-up Chase Kalisz of the United States claimed the gold in 4:09.42. American Jay Litherland made the podium with a silver in 4:10.28. The 21-year-old Australian Brendon Smith, who finished 4:09.27 in the qualifier, struck a bronze in 4:10.38.

"It is my lifelong dream. It is what everyone dreams of in the sport. I do feel like I let the US down in 2016, even though I swam faster here. The US has a proud legacy in the 400m individual medley. This was my redemption story," said Chase Kalisz, who won silver at Rio 2016.

"When the Games were postponed by 12 months it gave me an opportunity to better myself and be one year faster, one year stronger. I've gone through hell to get here, having to swim in the ocean for two months in Melbourne (because of lockdown restrictions due to Covid-19) in 10C or 12C water. It means so much," said Brendon.

In Sunday's semifinals, Adam Peaty of Britain led way in a bid to retain his men's 100m breaststroke Olympic title. Yan Zibei of China also made the final on Monday, July 26.

Zhang Yufei of China clocked 55.89 seconds to enter the women's 100m butterfly final as the first seed. Zhang will battle with Marie Wattel of France, Australian Emma McKeon and world record holder Sarah Sjostrom.

"I am satisfied that I am the one that swam sub-56 the most besides Sjostrom," said Zhang.

Zhang is keen to take another gold for China in the event, in which China has only won at the Olympic Games once in 1992 by Qian Hong.