Xavier Gourmelon was one of the first responders at the scene of the fatal crash as he was working at the nearby Malar fire station in Paris where he was the duty officer.
He has revealed he thought Princess Diana would live because she did not appear to have any significant injuries when he arrived at the Pont de l’Alma tunnel.
In his first interview since the crash, and having only spoken publicly once before – via video link during her inquest in 2007 – Mr Gourmelon described the scene and how he thought Diana would pull through.
Mr Gourmelon, 50, said: “I massaged her heart and a few seconds later she started breathing again.
“It was a relief of course because, as a first responder, you want to save lives — and that’s what I thought I had done.
“To be honest I thought she would live.
“As far as I knew when she was in the ambulance she was alive and I expected her to live.
“But I found out later she had died in hospital. It was very upsetting.
“I know now that there were serious internal injuries, but the whole episode is still very much in my mind. And the memory of that night will stay with me forever.”
The former firefighter said the 36-year-old princess was on the floor in the back of the car and was moving slightly when he arrived.
Mr Gourmelon gave her oxygen and stayed besides her as she was removed from the car wreckage.
He said: “I could see she had a slight injury to her right shoulder but, other than that, there was nothing significant.
“There was no blood on her at all.”
But when Princess Diana was placed on a stretcher, she suffered a cardiac arrest and stopped breathing.
He said Princess Diana’s final words to him were: “My God what’s happened”.
Mr Gourmelon said when he first arrived at the scene in the Pont de l’Alma tunnel in Paris, he was unaware of who the famous victims of the crash were.
He told The Sun: “The car was in a mess and we just dealt with it like any road accident.
“For me this was simply a banal traffic accident, one of many that emergency services have to deal with and it was the usual causes, speed and a drunk driver.
“We were very close to there and it took less than three minutes to reach it.
“My 10-man team was in two trucks and we were first to arrive.
“We got straight to work to see who needed help and who was alive.”
Mr Gourmelon said it was clear when he arrived at the scene that the driver of the car Henri Paul was dead and there was nothing that could be done for him.
Although the princess was taken to Pitie-Salpetriere hospital, she died at 4am after she suffered a ruptured blood vessel next to her heart causing her massive internal bleeding, as well as other internal injuries.
Diana, 36, Dodi Fayed, 42, and their chauffeur Henri Paul were killed when their Mercedes crashed in the Pont de l’Alma tunnel in Paris on August 31 1997.
The former firefighter’s interview comes as Princess Diana’s former butler Paul Burrell has made a visit to the site of the fatal crash for the first time.
Mr Gourmelon said he only felt able to give an interview about the horrific incident because he is no longer a firefighter.
It comes as Prince William and Harry, who were just 15 and 12 when their mother was killed, made a personal pilgrimage to view the floral tributes left in memory of their mother as Harry confessed “all of us lost somebody”.
William and Harry stopped to look at the heartfelt messages, pictures and flowers left in remembrance of Diana, Princess of Wales at the gates of her former home Kensington Palace.