The incident came to light when their maid visited the house for cleaning purposes in the morning, police said.
The assailants entered the house in sector 43 at around 4 am after breaking the iron grills, they said.
They first tied legs and hands of 75-year-old Mohan Chandra Tiwari and then injured his son Kapil and his wife Kavita criticallly when they offered resistance.
The intruders attacked Kapil, a chief engineer in Air India, and his wife with knives repeatedly, police said.
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Mohan Chandra Tiwari had retired from the All India Institute of Medical Science (AIIMS).
Kapil and Kavita were admitted in a private hospital where their condition was stated to be critical.
Gurgaon Police Commissioner Alok Mittal said police had got some clues and the attackers would be arrested soon.