* American outsider wins after crazy finale
* Olympic champion Miller-Uibo stumbles with gold in sight
* Teenager Nasser takes silver
* Favourite Felix fades to bronze (Adds Felix quotes)
By Mitch Phillips
LONDON, Aug 9 (Reuters) - American Phyllis Francis won a surprise World Championships 400 metres gold on Wednesday as Olympic champion Shaunae Miller-Uibo inexplicably stumbled when seemingly certain of victory in an extraordinary finale.
Francis looked out of the medals with 80 metres to go but maintained her form amid the carnage to post a personal best time of 49.92 seconds and take a shock gold that even after crossing the line she had no idea she had won.
Bahrain's Salwa Eid Naser, 19, claimed a brilliant silver in 50.06, her third national record this week, as defending champion Allyson Felix of the United States faded to get bronze in 50.08.
Bahamian Miller-Uibo, who famously dived over the line to pip Felix to Olympic gold last year, was clear with less than 20 metres left but as she tired and tied up she tripped on her own foot, stumbling almost to a standstill as her rivals stormed past.
It was incredible finish to a race that had appeared to be going to form for the first 300 metres. Felix looked her usual smooth self despite the sodden track and had made up the stagger on Francis, a lane outside her, by the end of the first bend.
However, she did not come off the final bend with her usual authority and Miller-Uibo surged ahead into the final straight.
The title seemed secure as Felix began to fade further but Miller-Uibo lost her form in the most dramatic style. Her legs seemed to stiffen, perhaps through cramp, and became entangled.
As she virtually stopped running, the fast-finishing Francis, Naser, Felix and fourth-placed Shericka Jackson of Jamaica all went past.
"I am so excited. It is such an amazing feeling, being world champion sounds pretty cool," said Francis, who finished fifth in the Rio Olympic final but took a gold in the 4 x 400m relay.
"This win has not hit me yet, but I guess it will tomorrow when I will wake up.
"Allyson and Shaunae are amazing finishers but 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."
Felix's bronze took her world championship medal haul to 14, matching the record of Jamaican sprinters Usain Bolt and Merlene Ottey. The American's tally includes nine golds and she could not disguise her frustration at not making it 10 - though she still has the 4x400m relay to come.
"I can't lie, I'm disappointed to lose one gold tonight but the championships is not over yet so we keep going," Felix said.
"But this was the one 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 under-estimate anyone."
Editing by Ed Osmond