Endurance athlete Diana Nyad has decided to end her swimming ultramarathon from Cuba to Florida about halfway through, as her face and body grew swollen from painful man o' war stings.
Medics warned another sting could be life-threatening.
Nyad team member Vanessa Linsley said the swimmer was disappointed and emotional.
The 62-year-old swimmer has completed at least 49 miles (79 kilometers) of the 103-mile (166-kilometer) passage of the treacherous Florida Straits.
Nyad tried last month, but had to quit after a serious asthma attack made it difficult to breathe.
Nyad suffered a second painful jellyfish sting Saturday night around the halfway point of her bid to make the 103-mile crossing from Cuba to Florida, but said she was intent on continuing the trip.
Nyad's face and eyes were affected and she was receiving medical treatment aboard a support vessel, according to online updates posted by her team.
"At this moment it appears that Diana wishes to continue," read a message sent on her Twitter account at the time.
A subsequent post on her website said an observer from the International Swim Federation says she can continue the swim if she has only been removed from the water for medical treatment and not to rest. It would be classified as a "staged swim" but still be a record if she completes it.
It was the second painful sting Nyad has experienced more than 24 hours into her attempt to break her own record for an open-water crossing without a shark cage by about a half-mile. Earlier Saturday, her assistants reported that she had swam 49 miles since setting out from a Havana marina the previous day.
She also got a visit from a curious shark, though her handlers downplayed the afternoon encounter.
"Around 1pm -- and don't everybody get excited here -- an Oceanic white tipped shark was spotted near Diana in the midst of the three boat flotilla. ... Rob MacDonald, one of (her safety) divers, swam towards the shark, where they faced off within 10 feet of one another," read a post on Nyad's website.
"I guess he thought I was more aggressive than him, and he turned in the other direction," MacDonald was quoted as saying.
Barracudas were also spotted in her general vicinity.
Without a cage to protect her, Nyad was relying on equipment surrounding her with an electrical field that is harmless but deters most sharks. Her divers are there to gently discourage any who make it through.
Nyad's run-in the previous night with a Portuguese Man o' War, described as "scary," left her with stings on her face, arms and side.
Complaining of difficulty breathing, she received oxygen and a steroid shot from her doctors and tread water while she recovered.
When she resumed swimming, her handlers noted that her stroke count had dropped to 48 per minute, down from 52-55, but as the day went on she picked up the pace.
"This afternoon -- it is stunning to actually witness -- Diana is swimming stronger and stronger," her website read. "Her strokes are up to 50 per minute, she is eating pasta, gobbling bananas, bits of peanut butter sandwiches, along with high-carb & high calorie liquid concoctions."
Her website reported that she seemed to revive significantly after consuming blended chicken soup.
The Los Angeles woman pauses every 45 and 90 minutes to rest and refuel on food that her assistants pass to her in the water, but without getting on the boat.
Not all encounters with marine life were unpleasant.
"At 7:10pm, on the crimson horizon of the Straits of Florida, ten pilot whales emerged in the distance just ahead of Diana," said a tweet.
Her team said a critical phase of the swim was beginning, as an observer on the boat said marathon swimmers are more likely to succeed if they make it through the second night.
Nyad said her strategy for enduring the roughly 60 hours it would take her to swim from Cuba to Florida, testing the limits of human exhaustion at the age of 62, is to focus on one partial goal after another.
"The mental approach has to be to parcel it out. No one could swim the whole thing while thinking about the whole thing," she said on her website. "Till I get a glimpse of the final shore I can't start obsessing on it -- it's too far away, too nebulous -- I don't know where it is. So I'm going to take it night by day: first get through the nighttime and then get through the daytime, and then the nighttime again, and so on."
Nyad is making her second attempt in as many months at the Cuba-Florida crossing, a lifelong dream that she first tried as a 28-year-old back in 1978, when she swam inside a steel shark cage for about 42 hours before ending the attempt.
A cageless attempt this August fell short 29 hours in when, gasping for breath, Nyad threw in the towel after an 11-hour asthma attack she blamed on a bad reaction to a new medicine.
But she said the experience did not convince her that she was physically not up to the challenge, and Nyad, who insisted the problems had nothing to do with her age, came to think of the aborted swim not as a failure but an unplanned dress rehearsal.
Though she had vowed there would be no second attempt, this week she flew to Cuba to take advantage of what she called a "magical window" of good weather and sea conditions.