Francis who could not believe her good fortune said: "It is amazing. I am so excited. It is such an amazing feeling. Being world champion sounds pretty cool. This win has not hit me yet, but I guess tomorrow when I will wake up.
"Allyson and Shaunae are amazing finishers, we are that talented. When I went down the home straight I just believed in myself and stayed patient. I just knew what I was capable of doing, so I stuck to my race model.
"At the finish line I was surprised, I thought I was second or third, but then they told me 'you are first'. That is crazy."
There was another surprise as 19-year-old Salwa Eid Naser of Bahrain (who has a Nigerian mother) took the sliver in 50.06seconds, which was a personal best and a National Record. She said: "I think I surprised my opponents and I even surprised myself. Coming to these championships I really did not think about a medal. After all the hard training I have been through, I just hoped to do something big. I was not really nervous before the race; just the same anybody is before the start.
"I was not chasing Felix, I was pushing myself till the very end and I did not even see what was going on in the last metres. Felix is my role model; I am following her on Instagram.
"I just want to thank my team and my family: 'I love you mama, I did it for you.' In the future I will also focus on the 200m, but it's 400m for now."
Alyson Felix, who equalled Usain Bolt’s record of winning 14 medals at these World Championships, as she picked up a bronze medal with a time of 50.08s, was disappointed not to have won despite being the favourite. She said: "I cannot lie, I am disappointed to lose one gold tonight but the championships is not over yet so we keep going But this was the race that mattered to me, the individual race, that is what it is about. So to come up short tonight is never fun. But I am happy for my team mate. At this level, you cannot underestimate anyone.
"I am not hurt, it is just the 400m, it is what it is. The conditions were the same for everyone, I am not sure if that was the factor tonight. It all seems wet to me.
"I still feel it is not over yet. I still did not give everything so I am excited to keep going until 2020."