After 11 hours in the water Wednesday, Australian athlete Chloe McCardel, 28, ended her attempt to swim from Havana, Cuba, to Key West due to severe jellyfish stings, her spokesman said.
McCardel, who did not wear a wetsuit, was pulled from the water about 9 p.m., ending her first try at the 103-mile swim without a shark cage -- leaving the feat still unaccomplished.
At 10:30 p.m., McCardel was on of her team's support boats headed to Key West.
"She will spend the next 24 hours recuperating before deciding on her plans going forward," Tim Stackpool wrote in a news release on behalf of www.chloemccardel.com.
Endurance swimmer Diana Nyad, 63, recently announced that she would make a fifth attempt at the Florida Straits crossing later this year. In August, she made it almost halfway from Cuba to Florida after 60 hours in the ocean, but quit after suffering jellyfish stings and enduring a lightning storm.
Nyad tweeted her commiseration Wednesday night.
"It's a tough night for Chloe McCardel, a superior swimmer and an exemplary spirit," Nyad wrote.
Fellow Australian Penny Palfrey, at 49, tried the crossing without a cage, too, in June 2012, and made it 79 miles before quitting.
Susan Maroney, of Australia, is the only person to make the Cuba to United States swim, completing it at age 22 in 25 hours in May 1997 -- but using a shark cage.
Both Nyad and McCardel swam under English Channel marathon rules, meaning the athlete cannot touch a support boat or hold onto anything during the swim.
McCardel, of Melbourne, had hoped to complete the swim in about 60 hours. Before starting out from Cuba in the morning, she arrived smiling and upbeat 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."
McCardel then jumped feet first into the water at 10 a.m. sharp. Her goal, the Keys, lay a little more than 100 miles to the northeast of Havana.
The sea off Havana was flat and glassy, precisely the ideal conditions that McCardel's science team had forecast.
"It is the hardest swim in the world today," McCardel had said 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 Associated Press contributed to this story.