Andrea Woodburn, one of the team members, said by phone that the British-born Australian marathon swimmer halted her effort around midnight about 42 kilometres south of Key West because of a strong ocean current working against her.
"She is fine," Woodburn said from Florida's Key West, without giving further details on Palfrey's condition. Woodburn said she had been in contact with Palfrey's boat and was told the swimmer had to get out of the water when she encountered the too-strong current in the Florida Straits.
Her team also tweeted that Palfrey had been pulled from the water but gave no further updates early Sunday. The waters in the area are notorious for fickle ocean currents, including the powerful Gulf Stream. But further details of whether she had encountered the Gulf Stream or a side eddy of that current or some other were not immediately available.
Earlier in the day, Palfrey had been swimming steady and strong and reported no physical complaints other than jellyfish stings more than halfway into the swim, according to her support crew.
At about 10 p.m. ET Saturday (0200 GMT) roughly 39 hours into the swim, the 49-year-old grandmother was 127.13 kilometres from her starting point at a marina in the Cuban capital, according to an online GPS tracking report. She was positioned at that time about 45.06 kilometres south, southwest of Key West.
Previously, her personal best was 108 kilometres when she swam between Little Cayman and Grand Cayman islands last year, according to Woodburn, part of her support team in the Keys.
Palfrey had reapplied sunscreen and grease to prevent chafing and said the water conditions had been excellent other than the extreme heat on the first two days of the swim. She said she had even spotted a few hammerhead sharks, along with dolphin pods. Crew members said she was in full control of the effort at the time, instructing team members who are accompanying her on kayaks and a catamaran as she kept up a torrid pace.
Hours earlier, Woodburn had said the conditions appeared ideal with bathwater-warm waters reported to be calm as the swimmer took aim for the Florida Keys. And the 20-year veteran of distance swimming has been no stranger to jellyfish stings, which forced her to abort two past swims in Hawaii.
Palfrey set off from Havana early Friday. A member of her crew had been tweeting to fans, while a webpage updated her location every 10 minutes or so based on data from a GPS device worn by the swimmer.
The daunting effort has been commonly reported as a 166-kilometre swim, however the GPS co-ordinates suggest it is more like 172 kilometre.
Multiple challenges loomed for Palfrey on her second day in the water Saturday, including the prospect of physical and mental fatigue and fending off dehydration, hypothermia and potentially dangerous marine life. At the rate she had been swimming, it had been estimated the crossing would have taken more than 56 hours to complete, slightly above her initial estimates. That would have put her on track for a Florida Keys' arrival sometime on Sunday if she had kept going.
Palfrey had been aiming for the record books to become the first person to swim from Cuba to the Keys without the aid of a shark cage. Instead she had been relying on equipment that surrounded her with an electrical field to deter the predators. Her support team consisted of more than a dozen navigators, handlers and medical personnel escorting her on the 13-metre catamaran Sealuver.
Woodburn said that Palfrey was hydrated on the swim every half-hour with a liquid concoction containing electrolytes and carbohydrates.
Australian Susie Maroney made the crossing in 1997 at age 22, but with a shark cage. American Diana Nyad made two unsuccessful cageless attempts last year on either side of her 62nd birthday, but had to call them off due to a debilitating asthma attack and painful Portuguese man o' war stings. She has announced plans to try again this summer.
Nyad's twitter feed displayed several messages focused on Palfrey's effort.
"I'm sure our team will learn from her crossing," one tweet reported.
Woodburn said that Saturday night Palfrey changed from a normal sport swimsuit to a porous, non-buoyant Lycra bodysuit to provide cover down to the wrists and ankles to fend off jellyfish stings.