Kolkata, Jan 9 : The medical condition of acclaimed classical singer Rashid Khan, who has been suffering from prostrate cancer and admitted to a private hospital since last month in Kolkata, turned critical on Tuesday after which he was put on ventilator support. The singer, 55, of the Rampur-Sahaswan Gharana, had undergone treatment at Tata Memorial Cancer Hospital. However, at a later stage, he preferred to continue with his treatment in Kolkata only. Sources close to him said that since the singer was admitted to the private hospital last month, he started responding to the treatment, and a team of doctors attending on him was happy to note the improvement in the medical condition. The sources said that despite ailment, there was hardly any change in Khan's routine, especially with regard to his daily practice of classical music in the hospital in Kolkata. His musical career started when he was 11-year-old. Khan's fusion and film numbers have also received accolades. /IANS