Tatjana Smith took South Africa's first gold of the Paris Olympics on Monday and added the 100 metres breaststroke title to the 200 metres she won at Tokyo 2021 under her maiden name of Schoenmaker.
China's world champion Tang Qianting took the silver medal and Ireland's Mona McSharry looked stunned, tears flowing, after an extraordinary three-way battle for bronze.
Tang led at the turn but Smith, in lane four but only fourth at that point, picked off her rivals and powered back to win in one minute 05.28 seconds.
That was 0.26 clear of Tang and made up for securing only silver in the event in Tokyo.
The winning time was slower than the 1:05.00 Smith had clocked in the heats.
She will also be a favourite for the breaststroke double by defending her 200 title to secure a place in the record books as her country's most decorated Olympian, equalling Chad Le Clos's four.
McSharry touched out third by the very slimmest of margins, beating Italy's Benedetta Pilato and world record holder and 2016 champion Lilly King of the United States both by 0.01 of a second.
The 23-year-old's medal was Ireland's first of the Games and there were plenty of her supporters in the stands.
"This isn't something that little me, dreaming of just going to the Olympics, really thought was possible," Smith said..
"I had a bad first 50, my goggles filled up with water a little bit.
"I started crying on the podium and I haven't really fully started going yet."
American Lydia Jacoby won the event at the Tokyo Olympics but failed to secure a ticket to Paris after finishing third in the U.S. trials with only the top two qualifying.
There has yet to be a repeat champion in the event.