Illusionist David Blaine, who is spending seven-days submerged in a water-filled sphere, is to receive medical treatment.
Blaine has special gloves to try and soothe the skin on his hands
Blaine's spokesman Pat Smith said the 33-year-old's peeling skin and overall condition was worrying doctors.
But he added Blaine was "determined" to complete the "human aquarium" stunt at Lincoln Center in New York.
A mask and an air line are keeping Blaine alive, and he is also getting liquid nutrition fed via a tube.
But prolonged submersion in water poses a number of hazards, including nerve damage, blackouts, sleep deprivation and skin problems.
Blaine has said his skin is causing him pain "like constant pins and needles" after five days in the acrylic sphere.
Mr Smith said: "Doctors are concerned that he's weakening. They are going to work with him through the weekend, trying to stabilize both his diet and his training regimen.
"His skin is peeling very badly on his hands. Those are our biggest concerns right now."
Blaine will end his challenge on Monday with an attempt to break the world record for holding breath under water, which currently stands at eight minutes, 58 seconds.
He has said for the record attempt he would come out of the sphere to be handcuffed, wrapped in 150lb (68kg) of metal chains and dropped back inside.
Blaine has undergone intensive training for the challenge, shedding 50lbs (23kg) in body weight since January to improve the efficiency with which his body uses oxygen.