The 98-year-old screen icon was admitted to suburban Khar-based Hinduja Hospital, a non-COVID-19 facility, on Sunday.
Kumar, who was diagnosed with bilateral pleural effusion -- a build-up of excess fluid between the layers of the pleura outside the lungs, underwent a successful pleural aspiration procedure on Wednesday.
The veteran actor was initially set to be discharged on Thursday but family friend Faisal Farooqui said it has been decided to let Kumar rest for a day at the hospital.
''He is stable and he is resting at the hospital. There was an option to be discharged today but the family and the doctors decided that we should let him rest one more day in the hospital,'' he said.
The Hindi cinema veteran, whose career spans over five decades with hits like ''Mughal-e-Azam'', ''Devdas'', ''Naya Daur'', and ''Ram Aur Shyam'', was admitted to the same hospital last month for a regular health check-up.
Kumar's last big screen appearance was the 1998 film ''Qila”.