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.
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 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."
American Emma Weyant took the silver medal 0.68 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 Katinka Hosszu, who has dominated the event in recent years and was defending champion, could finish only fifth.
The 32-year-old 'Iron Lady' was looking to become the second-oldest women’s swimming gold medallist in history, behind American Dara Torres, and she has three more chances in this Games.
'MEANS THE WORLD'
In the men's 400m medley, a confident Kalisz powered to victory as the Americans finally made their presence felt.
The silver medallist from Rio went one better in Tokyo with a time of 4:09.42, Litherland trailing him home 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."
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, with Canada taking silver, 3:09 behind the winners, and the United States in bronze position.
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 their previous record of 3:30.05 set in April 2018.
Cate Campbell has featured in all three of the relay victories.