Fell pays heavy prize for horse problems in Rome
World number one Fell, from Tavistock, lied in 23rd place after the fencing with 760 points accrued from 15 wins.
The Beijing silver medallist gained some ground in the pool by swimming a superb time of 2:16.30
Fell's time was the fastest among the Brits competing in the event and third fastest overall, pushing her up to 15th.
But then came a nightmare for the 26-year-old in the equestrian arena.
She jumped the obstacles out of order and her ride was terminated.
Fell scored just 92 points for a total of 2,020 and 36th overall going into the run/shoot.
Fell produced a strong run/shoot – her time of 12:31.20 was fourth fastest.
But she had left herself with too much to do and could only finish 34th with a points total of 4,376.
Fell had finished the previous World Cup event in Hungary as the top Briton.
Contrastingly, in Rome, she was behind all her international team-mates.
Mhairi Spence finished fifth overall with 5,464 points, Freyja Prentice (5,348 pts) was ninth and Katy Livingston (5,272 pts) claimed 11th place.
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