The 85-year-old grandfather of Jelena Djokovic (Novak's wife) was found unharmed and tied up in a cornfield. (AP)

The grandfather of Jelena Djokovic, the wife of Novak Djokovic, was found tied up in a field after unknown assailants broke into his house in central Serbia.

Djokovic's press team says in a statement on Thursday the family is in "complete shock over what happened" to 85-year-old Miloslav Radisavljevic.

Attackers reportedly broke into Radisavljevic's house, kidnapped him, and took him to a field several miles away.

Police said, without identifying Radisavljevic, that they found a man early Thursday tied to a power pole close to his house in Ljig, some 30 miles south of Belgrade.

The man told the police he was also robbed by the two masked men of about 220 euros ($260) and a cell phone. Police say the man had no visible injuries.

According to Serbian news outlet Blic, the kidnappers contacted Jelena's uncle, Radisavljevic's son, for a ransom of two million euros.