Besides a prior cardiac arrest, she also had infection of her lungs and abdomen, "as well as significant brain injury", Kelvin Loh, the hospital's chief executive officer, was quoted as saying by the Straits Times.
"The patient is currently struggling against the odds, and fighting for her life," he said.
He said a multi-disciplinary team of specialists has been working round-the-clock to treat her since her arrival Thursday. He said they were "doing everything possible to stabilise her condition over the next few days".
The 23-year-old victim was brutally beaten and raped by six men on a moving bus in Delhi Dec 16. She is fighting for her life with severe multiple intestinal, abdominal and other injuries.
The case shocked India, triggering protests by college students and calls for death penalty for the criminals. The six suspects have been arrested.