Nyad, 61, stroked through shoulder pain and floated on her back when asthma made it difficult for her to breathe on the attempt to swim from Cuba to Key West that she began Sunday.
She said she pictured herself emerging from the water onto the beach and vowed to doggy-paddle there, if that was what it took. By early Tuesday, trembling in the water, the record-setting marathon swimmer knew she had to stop, even though it meant giving up on her dream.
According to her Twitter feed, she was pulled from the water after swimming for 29 hours, though the exact distance she swam wasn't clear. The swim had been expected to take about 60 hours to cross 103 miles (166 kilometers).
"Sometimes the will is so strong. That's the whole point of this sport in general, that the mind is stronger than the body," she said after her support boat docked in Key West.
"I was shaking and freezing . and I thought, there's no mind over matter anymore. I was so depleted from the asthma," she said, crying in a white bathrobe before cheering supporters.
"It was so hard. I couldn't even swim. I couldn't be the swimmer I am," Nyad said, detailing the ailments that piled up as the waters got choppier. "I had severe asthma for 11 hours - I was taking 10 strokes and then going on my back and gasping. I had severe pain in my right shoulder that was so excruciating that every stroke I took from the third hour all the way through 30, I just winced every time."
Nyad had trained for and dreamed about the attempt for two years.
It was a dream deferred after she first tried to cross the Florida Straits in a shark cage as a 28-year-old in 1978. Then, she quit after 41 hours and 49 minutes in the water because of strong currents and rough weather that banged her around in the cage.
Now it's a dream unfulfilled. Nyad said she won't attempt try it again because she doesn't want to put her team through the ordeal of training again.
She attempted this swim without a shark cage, relying instead on an electrical field from equipment towed by kayakers to keep them at bay.
Her Twitter account reported she decided to end the swim herself, after "realizing the conditions of 5 to 10 knot winds and less than ideal currents."
An online chart plotting the swimmer's track showed the Gulf Stream currents pushing Nyad east of her intended course. Nyad had hoped to end her swim at the Key West marker for the southernmost point in the United States.
"When we got started, it was a flat, glassy sea. Two hours later, it started to chop up. Three hours in, I felt a twinge in my shoulder and it never went away," Nyad said. "I tried the fingers in different positions, I tried to swim in different ways, but I was just in wincing pain - and I thought, that's just how it's going to be."
Nyad said she tried taking puffs from an inhaler and medication to ease the asthma and the pain, but nothing helped.
Had the latest attempt been successful, Nyad would have broken her own record of 102.5 miles (165 kilometers) for an open-sea swim without a shark cage, set in 1979 when she stroked from the Bahamas to Florida.
Before beginning her swim Sunday night, Nyad told journalists she hoped her swim would inspire others her age to live active lives. She said she also hoped it could help improve understanding between Cold War rivals Cuba and the United States, even if just symbolically.
A Miami Beach lifeguard who completed a paddleboard crossing of the Florida Straits in June credited Nyad for inspiring her journey.
"I was inspired by her being a strong woman, attempting to do these amazing crossings no one had really done," Cynthia Aguilar said.
Propelling the board with her arms, Aguilar, 27, finished the 103.2-mile journey June 16, beginning about 14 miles off northern Cuba, in about 29 hours and 12 minutes.
She sympathized with Nyad's decision to quit, because she ended her own first attempt at the crossing last year in rough weather and strong currents.
"I learned that last year, that the water can change on you in a second," Aguilar said. "I admire her for going again. That's what life's about: If you fall, you try again. She went for it."
Diana Nyad Website: http://diananyad.com/