Tunisia and Japan celebrated unexpected gold medals 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 with a time of 3:43.36, with American Kieran Smith taking bronze.

Hafnaoui’s gold is only the fifth by a Tunisian athlete at the Olympics, but their third in swimming, and he was left stunned by his performance.

“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.

Phelps praise

Michael Phelps hailed Hafnaoui for his ‘unbelievable swim’ on Sunday, saying: “The difference between these Olympics and the past, in my opinion, is that every single person in the final has a chance of winning gold. It doesn’t matter if you are in lane one, eight or four, everyone is close.”

Australia won the 4x100 women’s freestyle relay for the third time in a row, beating their own world record by three seconds and helping take the sting out of a disappointing first day of finals in the pool.

The quartet of sisters Bronte and Cate Campbell, Meg Harris and Emma McKeon took 0.36 seconds off their previous best of 3:30.05 set in April 2018, with a world record of 3:29.69.

Gold-medal winning skateboarder Yuto Horigome of Japan in action on Sunday. Getty Images

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 team mates and Japanese officials to roar her home as she delivered gold for the hosts in a time of 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.”

In the men’s 400m medley, a confident Kalisz powered to victory as the Americans finally made their presence felt.

Biles qualifies

Simone Biles qualified for the finals of all six of her gymnastics events on Sunday.