Dream come true for duo

TOKYO • Tunisia and Japan celebrated unexpected golds on the opening day of swimming medal events before normal service was resumed with the Australian women's 4x100m freestyle relay team smashing their own world record on the way to the title in Tokyo.

Chase Kalisz settled American nerves by delivering the country's first gold of these Games, winning the men's 400m medley as part of a US one-two with Jay Litherland.

On a day of surprises, Tunisian teenager Ahmed Hafnaoui pulled off the biggest shock of all with a stunning victory in the men's 400m freestyle.

The 18-year-old, swimming in the outside lane as the slowest qualifier, produced a blistering finish to pip Australia's Jack McLoughlin to gold in 3min 43.36sec, with American Kieran Smith taking bronze.

Hafnaoui was so shocked to see his name in the gold medal position that he did not believe his eyes but after letting his stunning win at the Tokyo Olympics sink in, he said he hoped he made his family proud.

"I believe it when I touched the wall... I was so surprised I didn't accept that," he told a news conference. "I dedicate (the gold) to all my family... I wish they are proud of me."

His gold medal is only the fifth won by a Tunisian athlete at the Olympics, but their third in swimming.

"I just can't believe it. It's a dream and it became true. It was great. It was my best race ever," he said.

While Hafnaoui's coach celebrated the victory by leaping around the edge of the pool, it was hard not to wonder what the scenes would have been like had Yui Ohashi's victory in the women's 400m medley come in front of a home crowd.

It was left to her teammates and Japanese officials to roar her home as she delivered gold for the hosts in 4:32.08.

"It doesn't feel real. It is like a dream for me," she said. "I couldn't go to the Rio Olympics, so for the past five years this became a big dream for me. This accomplishment is amazing."

American Emma Weyant took the silver medal 0.68sec behind and compatriot Hali Flickinger picked up the bronze.

Ohashi pulled away from Flickinger in the breaststroke leg and went into the freestyle with a lead of 1.99 seconds, giving her a comfortable cushion to hold off Weyant's late surge.

Hungarian "Iron Lady" Katinka Hosszu, who has dominated the event in recent years and was defending champion, could finish only fifth. The 32-year-old was looking to become the second-oldest women's swimming gold medallist, behind American Dara Torres, and she has three more chances in these Games.

In the men's 400m medley, a confident Kalisz powered to victory as the Americans made their presence felt. The silver medallist from Rio went one better in Tokyo with 4:09.42, Litherland trailing home 0.86 behind. Australia's Brendon Smith was a further tenth of a second back taking bronze.

"It means the world. This is the last thing that I really wanted to accomplish in my swimming career," said the 27-year-old. "It was something that was a dream of mine for as long as I could remember. I can't believe it."

Japan's Daiya Seto, the pre-Games favourite for gold, had failed to qualify from Saturday's heats.

While it was a disappointing day for Australia in the men's events, the women set a world record of 3:29.69 in the 4x100m freestyle relay - the first at these Games - with Canada taking silver, 3.09sec behind the winners, and the United States the bronze.

The quartet of sisters Bronte and Cate Campbell, Meg Harris and Emma McKeon ensured a third straight gold in the event for Australia, taking 0.36 off the previous record of 3:30.05 set in April 2018.

Cate Campbell has featured in all three of the relay victories.

AGENCE FRANCE-PRESSE, REUTERS