Swimming four lengths does not sound like a particularly impressive achievement. But when those lengths add up to nearly 134 miles (215km) in the open sea, and when they have been completed by a breast cancer survivor a year after treatment, they look more like an extraordinary world first.

Over 54 gruelling hours from midnight on Sunday until dawn on Tuesday, the American ultra long-distance swimmer Sarah Thomas, 37, became the first person to swim the Channel four times without stopping. Thomas dedicated her achievement to fellow breast cancer survivors.

Thomas’s feat was accomplished in the face of strong tides that extended a route that had originally been expected to cover 84 miles, far less than the distance she eventually completed. The British endurance swimmer Lewis Pugh described the achievement as “extraordinary” and “superhuman”.

She was diagnosed with an aggressive form of breast cancer in November 2017 and underwent chemotherapy, surgery and radiation. Thomas’s support team said she used swimming as a means of coping.

In a Facebook post, a member of her team said the conditions on the final leg were “dark, windy and choppy”.

They added: “This is supposed to be the most challenging leg of the swim so please keep up the good vibes you’ve all been sending her way.”

Thomas celebrated her success with champagne and chocolates. An official observer said she had tested the limits of endurance. “It is amazing, absolutely inspirational. At the end we were very emotional,” Kevin Murphy said.

Thomas told the BBC: “I just can’t believe we did it. I’m pretty tired right now.” She said she planned to sleep for the rest of the day.

Before the swim, Thomas admitted she was scared of the challenge. “I’ve been waiting for this swim for over two years now and have fought so hard to get here. Am I 100%? No,” she said. “But I’m the best that I can be right now, with what I’ve been through, with more fire and fight than ever.”

Thomas, whose achievement puts her ahead of the four people to have previously completed the Channel crossing three times without stopping, said the swim was dedicated to “all the survivors out there”.

Thomas completed her first open-water event in 2007 – a 10km swim in her home state of Colorado – and went on to become the overall champion of the race. In 2016, she also set a distance record of 128.7km in 56 hours, non-stop, across Lake Powell in the US.

“This is for those of us who have prayed for our lives, who have wondered with despair about what comes next, and have battled through pain and fear to overcome,” she said.

“This is for those of you just starting your cancer journey and those of you who are thriving with cancer kicked firmly into the past, and for everyone in between.”

Thomas’s mother, Becky Baxter, said the attempt had been “scary” at times because her daughter was unable to keep down liquid on the first day. She had been suffering with the effects of the salt water, which had left her throat and mouth sore.

“This one was by far the scariest one and right up until she landed at Shakespeare Beach in Dover we didn’t know for sure that she would make it,” Baxter told BBC Radio 4.

“She had a lot of trouble with stomach ache and she did lose her lunch several times. And we were a little worried towards the end of the first day, during that night, because she was not able to hold anything down – water, anything. But we slowly built her back up.”

She said Thomas had sustained herself during the swim by drinking a bottle of carbohydrate shake every 30 minutes. “It is tied to a rope and we are on the boat. She drinks a third of that bottle in 10-15 seconds and then she takes off again. Sometimes she’ll eat solid foods, but we had a lot of trouble finding something that agreed with her on this trip,” Baxter said.

“She is a freak of nature. She really had to dig deep to finish this. She could have quit many, many times. There were several obstacles, but she never quits.

“In the last 400 metres into the beach, she was caught in the tide and was just not going anywhere. And so we had to do some screaming and yelling and get her to dig deep and she found it after all that time. I’m pretty proud of her.”