Damir Dokic's lawyer, Bosiljka Djukic, told The Associated Press on Thursday that she had been informed by Dokic's wife that he was no longer taking any food, water or medicines in prison. He is serving his sentence in the northern town of Sremska Mitrovica.
"I am very worried," Djukic said.
The 50-year-old Dokic was arrested in May after reportedly saying he would blow up Australian ambassador Claire Birgin's car over the negative articles in the Australian press in which Jelena Dokic accused him of beating her.
Upon arrest, police also found a large number of weapons in Dokic's home.
Dokic had threatened to fire a rocket launcher at Birgin's car, but later said the statements were made in anger. He also has admitted having beaten his daughter.
Dokic was charged with "endangering the security" of the Australian ambassador and unlawful possession of weapons. His 15-month sentence was confirmed in a retrial that was held after an appeal.
The official Tanjug news agency reported that Dokic was angered by a six-month suspended sentence in a similar case this week against a nationalist who had threatened a liberal politician and a journalist.
The 26-year-old Jelena Dokic, born in the former Yugoslavia, migrated to Australia with her family as a child and represented her adopted country at the 2000 Olympics.
A Wimbledon semifinalist in 2000, Dokic renounced her Australian ties in 2001 to move back to Serbia, then decided to compete for Australia again in 2006.