KEY WEST, Fla. — The currents in the Florida Straits finally proved stronger than the determination that had pushed Diana Nyad across vast stretches of open water before.

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. She swam right through a smack of stinging jellyfish. But 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. Later Tuesday, Nyad said her captain told her she had roughly 53 miles to go when she stopped. 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.”