Ahmed Hafnaoui scored a stunning upset, Chase Kalisz got the United States on the gold medal trail, Yui Ohashi brought joy to Japan and Australia's women broke new ground in the 4x100 metres freestyle in a blockbuster morning swimming session at Tokyo 2020.
The first medals in the pool were up for grabs on Sunday and a thrilling couple of hours of racing ensued at the Tokyo Aquatics Centre.
While there was plenty to celebrate, the morning belonged to a teenage rising star from Tunisia.
HAFNAOUI MAKES HISTORY
At just 18-years-old, Hafnaoui – who only just made it into the final by clocking the eighth fastest time in the heats – upset all the odds to take out the men's 400m freestyle gold.
Battling it out with Australia's Jack McLoughlin down the final 50m, Hafnaoui touched home first in a time of 3:43.36, shaving 2.32 seconds off his personal best time set on Saturday.
He is now Tunisia's second-ever swimming Olympics gold medallist and the first African from any nation to win gold in the event.
"I just can't believe it, it's a dream it came true," he told Eurosport. "It's about racing not the time, you see competitors and try to get yourself better than them. It was great, it was my best race ever."
KALISZ GOES ONE BETTER AS OSASHI TASTES HOME SUCCESS
Four years ago, Kalisz earned a silver medal in the men's 400m individual medley but he moved up a step on the podium in Tokyo to win USA's first gold of the Games.
The field for this race was blown wide open when favourite Daiya Seto failed to make it out of the heats and Kalisz led an American one-two from Jay Litherland, with Australia's Brendon Smith taking the bronze.
Seto may have missed out on the chance to race for gold on Sunday, but Japanese compatriot Ohashi won the hosts' first gold in the pool of the Games in the women's 400m IM.
Ohashi's time of 4:32.08 was enough to hold off American Emma Weyant despite tiring as the race neared its conclusion.
AUSSIES FREESTYLE TO NEW GROUND
The Australian quartet of Bronte Campbell, Meg Harris, Emma McKeon and Cate Campbell lived up to their billing in the women's 4x100m free.
Comfortably the favourites for success, the Aussies clocked a new world record time of 3:29.69 (the fourth time in a row Australia has set the world benchmark), the first time any foursome has gone under 3:30.
Elsewhere on Sunday, Adam Peaty continued his seemingly unstoppable march towards gold by going fastest in the men's 100m breaststroke semi-finals in a time of 57.63.
And in the wide-open women's 100m butterfly, China's Zhang Yufei qualified fastest for the finals with a time of 55.89.