A 62-year-old American woman is making good progress on her latest attempt to become the first person to swim from Cuba to the US without the protection of a shark cage, her support team says.
Diana Nyad suffered several jellyfish stings and hit storms, but is continuing her 166km (103 mile) swim.
She left the Cuban capital, Havana, on Saturday and aims to reach Florida on Tuesday, the day before she turns 63.
She is relying on an electronic shield as a shark deterrent.
It is her fourth attempt. Nyad first attempted the swim in 1978.
Her second attempt was cut short by shoulder pain and an asthma attack.
And the long-distance swimmer was forced to abort her most recent attempt last September after potentially deadly jellyfish stings.
She had to bring the latest attempt forward by 16 hours because of bad weather.
Nyad's team said on Monday the conditions were favourable.
"Her stroke looks good and we're moving in the right direction," Mark Sollinger told NBC in a call from a boat trailing the swimmer.
Earlier on the team's blog Nyad's supporters wrote that she had covered about a third of the distance to Florida in the first 35 hours.
The team described the first day as "awesome", with calm flat waters and Nyad maintaining a pace of 50 strokes a minute.
But the second night had been difficult, they said, with Nyad suffering multiple jellyfish stings - on her lips, forehead, hands, and neck - some of them from box jellyfish.
And in a post at 02:30 EDT (06:30 GMT) on Monday, they wrote that "all hell broke loose" as a storm blew up around the flotilla, bringing lightning and rain, and pushing Nyad eastwards.
The team was looking for a way through the storm, said the blog, but Nyad was "safe, feeling strong and is now swimming again".
Nyad is now wearing a special head-to-toe swimming suit to wear at night to minimise the threat from jellyfish.
She has also added jellyfish experts and detectors to her support team - alongside divers whose task is to protect the swimmer from sharks, reports the BBC's Sarah Rainsford in Havana.
A team of divers is on constant alert to plunge in and guide sharks away themselves, our correspondent says.
Before the swim, Nyad said she felt "excited", but that she was aware of the dangers.
"It's not perfect, you can see the white caps (on the waves). But I feel strong and... hopefully it will be calm so we can make it across before the big east wind starts coming in.
"I feel really excited. I respect this. I know how difficult it is - there is a reason no-one's ever done it. But I'm prepared. I may suffer some, but I'm prepared for that too."
Nyad's team expects the swim to take about 60 hours. She takes some short breaks to rest, hydrate and eat foods like peanut butter, which provide energy.