Tunisia's Ayoub El Hefnaoui made a big surprise on the first day of the swimming medals at the Tokyo Olympics, winning the 400m freestyle gold, while Yui Ohashi gave Japan an unexpected gold in the women's 400m individual medley on Sunday.

American Chase Kalish won the gold in the 400 meters medley, beating his compatriot Jay Letherland.

The coronation ceremony of Tunisian swimmer Ayoub El Hefnawy with the first Arab gold medal in the #Tokyo_Olympics after winning the 400m race #Tokyo2020 #Tokyo2020 #Olympics#Al Kass Channels pic.twitter.com/Fj87erQfRA

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The Australian women broke the world time of the relay 4 in 100 meters freestyle after achieving 3 minutes and 29.69 seconds to win the gold, while Canada won the silver and America the bronze medal.

The 18-year-old Tunisian swimmer was the slowest among the finalists and started the race in the last lane, but he shone in the last meters and beat Australian Jack McLoughlin, to take the gold in 3 minutes and 43.36 seconds, while the American Kieran Smith won the bronze.

Urgent: Tunisian swimmer Ayoub El Hefnawy achieves the first Arab gold medal in the #Tokyo_Olympics after winning the 400-meter race #Tokyo2020 #Tokyo2020 #Olympics#Al Kass Channels pic.twitter.com/NlEzH3ZpaQ

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This is Tunisia's fifth gold in its history in the Olympics, but it is the third in swimming, and El Hefnaoui was stunned after his performance.

"I can't believe it. It's a dream come true," he said.

Japanese surprise

There were no fans to cheer Ohashi, but her teammates and Japanese officials celebrated after the amazing performance to win the gold with a time of 4 minutes 32.08 seconds.

American Emma Wyant took the silver with a time of 4 minutes 32.76 seconds, while her compatriot Haley Fleckinger took the bronze with a time of 4 minutes 34.90 seconds.

Hungarian title holder Katinka Hossou, who has dominated the race in recent years, settled for fifth place.

Ohashi overcame Flickinger in the breaststroke stage and entered the final stage of freestyle swimming with a difference of 1.99 seconds, which helped her to withstand the attempts of Wiant.

After winning the silver in the 400 meters medley at the Rio Olympics, Kalish improved and grabbed the gold with a time of 4 minutes and 9.42 seconds, for America to dominate the first two places after Jay Letherland took the

silver by 0.86 seconds.

Australian Brendon Smith took the bronze.

Calisch, 27, led the race after the noon swim, with France's Leon Marchand in first place after the butterfly stage.

Calisch competed strongly with New Zealander Louis Clairbert during the breaststroke stage, but his opponent fell sharply in the freestyle stage and finished seventh in the end.

"It means everything," Kalish said. "This is the last thing I really wanted to achieve in my swimming career. It's something I've been dreaming of for a long time. I can't believe it."