LONDON — An American cancer survivor has become the first person to swim the English Channel, which separates England and France, four times nonstop.

Sarah Thomas set off from Dover just past midnight on Sunday. She completed her fourth lap of the channel 54 hours and 13 minutes later on Tuesday morning, after 133 miles of swimming.

The exhausted swimmer lay down on the pebbled beach after coming out of the water, and was then handed some M&Ms and a glass of bubbly.

American Sarah Thomas crosses the English Channel for the fourth consecutive time on Sept. 17, 2019. Jon Washer

“Yes, I'm scared. The Channel can do a lot of crazy things and for this to happen, we're going to need some luck,” wrote Thomas on Facebook on Saturday before setting off.

“But, hopefully between the preparation I've done and the experience of my crew and boat captain, we can get this done together.”

Thomas, who lives in Colorado, was diagnosed with breast cancer less than two years ago and underwent treatment in the months that followed. Before her diagnosis, Thomas swam long distances and as soon as her health allowed she got back into the water.

"I've been waiting for this swim for over two years now and have fought so hard to get here. Am I 100%? No. But I'm the best that I can be right now, with what I've been through, with more fire and fight than ever," she wrote in her Facebook post dedicating this swim to "all the survivors out there."

The tracker map of Sarah Thomas' four consecutive channel crossings. https://track.rs/ssthomas3

A boat equipped with GPS systems and several computers accompanied Thomas throughout her journey, monitoring wind, tides and currents. A crew that included her mother, husband and several swimmers who joined her at points during her journey, cheered her on and ensured she stayed nourished during the trip.

Thomas would take breaks every 30 to 60 minutes while swimming to refuel with protein drinks and her favorite snack, M&Ms, according to Eddie Spelling, the captain of Thomas’s escort vessel, Anastasia.

Sarah Thomas became the first person to swim across the English Channel four consecutive times on Tuesday. Jon Washer

“She’s very tired and rightly so,” said Spelling in a phone interview with NBC News not long after Thomas completed her last lap. “America should be proud. It’s the best thing that I’ve ever seen.”

Documentary film maker Jon Washer, who is making a film about Thomas' journey, posted a photo on Twitter of the swimmer catching her breath on the beach after emerging from the water.

"I just can't believe we did it,” Thomas told the BBC after she landed. "I'm really just pretty numb. There was a lot of people on the beach to meet me and wish me well and it was really nice of them, but I feel just mostly stunned."

Swimming the Channel even once is no easy feat. Even experienced swimmers train long and hard to make the trip across the cold, busy shipping lane with strong currents. The first recorded trip across the channel took place at the end of the 19th century by Matthew Webb, whose trip lasted 21 hours and 45 minutes.