Tokyo: Tunisia's Ahmed Hafnaoui pulled off a major shock on the opening day of the Tokyo Olympics swimming medal events on Sunday with a stunning victory in the men's 400m freestyle while the hosts celebrated a surprise gold for Yui Ohashi in the women's 400m medley.
Chase Kalisz powered to victory in the men's 400m medley as part of a United States one-two with Jay Litherland taking silver.
Australia's women set a world record of 3:29.69 in the 4x100m freestyle relay, with Canada taking silver and the United States bronze.
The 18-year-old Hafnaoui, in the outside lane after being the slowest in qualifying, produced a blistering finish to pip Australia's Jack McLoughlin to gold with a time of 3:43.36, with U.S. swimmer 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.
There was no crowd in the arena to cheer on Ohashi but her team mates and Japanese officials celebrated after she produced an immaculate swim to win gold in a time of 4:32.08.
American Emma Weyant took the silver medal 0.68 behind and compatriot Hali Flickinger picked up the bronze in 4:34.90.
Hungarian Katinka Hosszu, who has dominated the event in recent years and was the defending champion, could finish only fifth.
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
Kalisz, the 400 medley silver medallist in Rio, went one better in Tokyo with a time of 4:09.42 with Litherland 0.86 behind. Australia's Brendon Smith was a further tenth of a second back taking bronze.
Kalisz, 27, grabbed the lead on the first length of the backstroke after France's Leon Marchand had led after the butterfly leg and never looked back.
He battled with New Zealand's Lewis Clareburt through the breaststroke but the Kiwi faded badly in the freestyle and finished seventh. "It means the world. This is the last thing that I really wanted to accomplish in my swimming career," said Kalisz.
"It was something that was a dream of mine for as long as I could remember. I can't believe it."