A run along the Chicago lakefront turned into a rescue when graduate student Fiona Cantorna heard someone in Lake Michigan screaming for help and jumped in.

Guy Fuller said his friend was in the water last Thursday, fighting to stay afloat, while officers were already at the scene but no flotation device was in sight.

“When the police can’t help you with something like this, there’s a part of you that says jump in. The heart says jump in. The mind says you can’t do it,” Fuller said to NBC 5. “I still have flashbacks, watching the near-drowning.”

Cantorna said she did not hesitate.

“I saw that she was drowning, and I knew that ok, this is what you have to do to save this person, and I think that my rational side took over, and I think, I think it was that switch,” she said.

A former competitive swimmer and one-time lifeguard, Cantorna said instinct took over and her training kicked in.

“I just didn’t think. I think I honestly went on autopilot,” she said. “There was a lot of adrenaline in that moment. It was cold, but I didn’t necessarily feel it, I guess.”

After pulling the woman to safety, Cantorna warmed up in the back of a police car.

Fuller, who said he has known the woman since she was a child, said watching a stranger save her changed how he felt about people.

“This was divine intervention, no question about it, and it just restores your faith in humanity,” he said. “We hear so many bad stories, to hear a story like this really restores your faith in mankind, and I am just so thankful for Fiona and so appreciative of her.”

Read more from NBC 5.