Singh has been put on ventilator after he underwent a critical surgery.
"He is still in the ICU (Intensive Care Unit) and under observation. His health is stable yet critrical, " Lilavati Hospital spokesperson Sudhir Dagaonkar told media.
The 70-year-old singer was to perform with Pakistani Ghazal singer Ghulam Ali in Mumbai on Friday evening.
Singh has suffered a heart attack in January 1998.
He had to be hospitalized in 2007 with blood circulation problems.