Veteran Swedish sprint star Sarah Sjoestroem said she was "so in the zone" after winning the women's 100m freestyle on Wednesday to secure her first gold over the distance in a storied career.
The 30-year-old world record holder touched first in a raucous La Defense Arena in 52.16secs ahead of American Torri Huske and Hong Kong's Siobhan Haughey.
Australia's Mollie O'Callaghan, the 200m champion, came fourth.
"I think my reaction said everything. I didn't really know where I was exactly when I finished. It took a few seconds before I saw that I won," said Sjoestroem, best known as a 50m free and fly swimmer who is competing in her fifth Olympics.
"I just felt like I had a really good race and I was so in my zone, 100 per cent focused on all the details that I've been working on.
"That's always been my challenge when it comes to the 100 free because I really need to focus on the breathing pattern.
"When I do the 50 free there's no problem. I just put my head down and I spin my arms and I go fast."
The 2016 Olympics 100m butterfly gold medallist initially wasn't planning to enter the 100m in Paris.
Despite winning four Olympic medals and 25 at world championships, she had never claimed a gold in the 100m freestyle until now.
"I didn't think I would swim the 100 free, honestly. Freestyle relay the first day I told my coach no, I don't think the 100 free is for me," she said.
"I want to do the 50 free, I want to rest until the 50. He was like no way, you need to go out there and see what you can do no matter the outcome.
"I'm super proud of myself that I tried this," she added.
Huske went out hard and turned in the lead at halfway.
O'Callaghan is renowned as a fast finisher and was seen as favourite, but she failed to bring it home with Sjoestroem winning by a fingertip.
"I was feeling nervous, not having a lot of sleep for the past few days. I tried really hard to manage myself and get up for this," said a disappointed O'Callaghan.
"I knew it was going to be a tough race from the start. We're all really close. It's one of those races, everything counts.
"If you stuff something up, it really costs you. At the end of the day I've just got to suck it up and wait another four years."