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Olympic swimming took a dive into the unknown on Thursday as men and women competed against each other for the first time in the new 4x100m mixed medley relay in Tokyo.
Britain, led by Adam Peaty, and world record holders China won the two heats of the inaugural competition, which has its final on Saturday and makes its debut in Japan as part of a drive to boost gender equality.
Britain will be the favourites after their time of 3min 38.75sec was more than two seconds ahead of the rest of the field and only a shade outside China's world record 3:38.41 set last year.
"I haven't been part of many mixed relays so it is fun to get out with guys and girls," said Australia's Brianna Throssell.
"The lead changes so often and no-one knows each other's tactics. It was really fun to get out here and do the heat for Australia."
Swimming is one of seven sports to launch mixed events at this year's Olympics, with athletics, archery, judo, shooting, table-tennis and triathlon also making the move.
The mixed medley will be familiar to elite swimmers, having already appeared at the world championships in 2015, 2017, and 2019.
Australia are the current world champions after Britain won in 2015 before the United States took the title two years later.
"It's a popular event at every championship so for it to come to the Olympics shows what swimming can be," said Peaty.
"It doesn't have to stay the same as it's been for the last 100 years. It can develop and evolve to what the modern world is.
"It's a new race and it's entertaining. You might be leading by 100 then someone else takes over which is the fun of it."
Teams are made up of two women and two men, with each of the four swimmers allocated to one of the four traditional medley strokes -– backstroke, breaststroke, butterfly and freestyle.
The order of strokes must remain the same but teams decide which athletes, male or female, swim in each, meaning the race is a tactical battle as well as a physical one.
While the backstroke and butterfly were more evenly distributed in the heats, nearly all teams picked men to swim breaststroke and women for the free. Only Petra Halmai bucked the trend by swimming breaststroke for Hungary.
It remains to be seen whether others are called up for the final on Saturday, with America having the option of speed demon Caeleb Dressel in either the free or fly.
Britain, China, America and Australia will be joined in the final by the Netherlands, Italy, the Russian Olympic Committee team and Israel.