Diana Nyad, 62, hit the water on Saturday, one day earlier than expected.
The 103-mile swim was set to take an estimated 60 hours to complete. Nyad was reported to be comfortable and confidant Sunday afternoon, going at a rate of about 50 strokes per minute. She takes periodic short breaks to rest, hydrate and eat high-energy foods like peanut butter.
An asthma attack and painful jellyfish stings forced Nyad to quit earlier attempts. Jellyfish presented a similar hurdle this time around as well, despite a new-and-improved bodysuit she hoped would offer better protection. She was stung four times overnight on the neck, lips, hand and forehead, according to members of her 50-person crew, who updated fans via social media.
At 18 hours into the swim, she had traveled 21.7 miles from Havana. Choppy seas were calmer, and her team reported only a light wind.
If she succeeds this time, she will arrive in Florida on Tuesday, one day before her 63rd birthday.
The Associated Press contributed to this report.