John Tomic struck his son's training partner with his head only because his own arms were being held by Drouet and he had to protect himself from falling over, his lawyer said.
"There was some pushing and he acted in legitimate self-defence," Dieguez said.
Tomic opted for a court hearing rather than paying a fine, she told reporters, while Drouet earlier claimed he had been "treated like a dog" by the coach.
Bernard Tomic's father, who is also his coach, was arrested several hours after the fight in the central avenue Paseo de la Castellana and was jailed for a period before being released, police said earlier.
The player has made a poor start to this year's European clay court season and lost in straight sets to Czech veteran Radek Stepanek in the first round of the Madrid Masters.
In a statement, Tennis Australia said the allegations were "very concerning".
"We are working closely with ATP officials who are investigating the incident and are unable to comment further until the full facts are known," it said.
In France, L'Equipe newspaper said Drouet sustained a broken nose, stitches to a cut above his eye and bruising to the back of his neck.
L'Equipe quoted Bernard Tomic as saying that he had been with his physical trainer at the time of the incident and had seen or heard nothing.
"It's a very odd situation," he was quoted as saying by the paper. "I wanted to talk to Thomas, but he was in hospital. I haven't seen my father since then."
Drouet told NewsLtd the incident followed months of mistreatment by John Tomic.
He also claimed he saw Tomic Sr punch Bernard in the mouth during a practice session last Tuesday, leaving the player in tears.
Drouet said Tomic Sr demanded he buy him a carton of milk as they were waiting to go to the airport.
He said he replied that he didn't have time before being abused, sacked and ultimately attacked by John Tomic.
"He spat in my face, then walked away, and said he wouldn't pay me any more," Drouet told NewsLtd before the hearing.
"I said, 'OK, John, you are a real man. That's fine. Bernard will pay me'.
"And he headbutted me.
"Every day he treat me like a dog for six months, with disrespect and then this. But he need to be off the ATP (tour)."
Bernard Tomic last year created more headlines off the court than on it, including run-ins with police in his sports car and claims he was not trying hard enough.
The 20-year-old recently was recalled to Australia's Davis Cup team after captain Patrick Rafter disciplined him for a lack of professionalism following admissions that he gave less than full effort in matches at New York and Shanghai last year.
Tomic won both his singles matches and clinched victory for Australia in their Davis Cup Asia-Oceania Group One second-round tie against Uzbekistan last month.
- AFP-AAP
