Australian endurance swimmer Chloe McCardel ended her attempt to swim from Cuba to Florida after 11 hours in the water when she was stung by a jellyfish.
It was the fifth failure involving three women swimmers who have tried to make the marathon swim since 2011. Jellyfish stings and strong currents have been the main impediments.
Her support team released a statement saying that Ms McCardel last night stopped her effort to become the first person to swim across the Straits of Florida nonstop without a shark cage "due to a severe debilitating jellyfish sting".
It said she had been taken on to one of her support vessels and was sailing to Key West. She would need 24 hours to recuperate, it said.
Diana Nyad, an endurance athlete who failed three times to make the same crossing and has said she would like to take another shot this summer, tweeted her commiseration.
"It's a tough night for Chloe McCardel, a superior swimmer and an exemplary spirit," she wrote.
Ms McCardel, a 28-year-old from Melbourne, had hoped to complete the swim in about 60 hours.
Before starting out from Cuba in the morning, a smiling, upbeat Ms McCardel had arrived in a pink 1950s Chevy convertible at a rocky jetty in western Havana
"As confident as I can be. I think it's all going to work out well," she said of her chances. "It'll be tough, though."
She then jumped feet first into the water at 10am local time, with her goal, the Florida Keys, a little more than 100 miles to the north-east of Havana.
The strait has been busy for the last three summers, with fellow marathon swimmers Ms Nyad and Penny Palfrey making four failed attempts at the crossing between them since 2011.
"It is the hardest swim in the world today," Ms McCardel had said on Tuesday at a news conference.
"No one has been able to achieve this. It's possibly harder than winning the World Cup or getting a gold medal."
The challenge also outstripped by far, at least in terms of distance, anything she had done before.
Ms McCardel, who has twice made a double crossing of the English Channel, said the most time she had spent in the water continuously was 25 hours.
She swam under English Channel Marathon rules, which meant she could not touch her support boat or hold on to anything.
Nor was she allowed to wear a full-body wetsuit, which would have helped protect against exposure and jellyfish stings.