But the 49-year-old, who said he acted in self-defence when he butted Thomas Drouet in the face and broke his nose, will not have to serve jail time as his sentence is less than two years, a court official said.
"In Spain nobody goes to jail unless their sentence is at least two years," the official told Reuters, adding that Tomic, who was also ordered to pay costs, had five days in which to appeal against the ruling.
A burly former taxi driver born in Bosnia, Tomic tangled with Drouet on the central Paseo de la Castellana during the Madrid Masters in which Bernard was competing and the incident led to the older Tomic being suspended from ATP events.
Australia's Herald Sun newspaper quoted Drouet as saying that Tomic had treated him like a "dog" for years.
Shortly before the head-butting incident, Tomic had told him he was sacked and would not be getting paid.
"I thought he was going to say sorry for the verbal abuse, but no," Drouet told the paper.
"He spat in my face, then walked away and said he wouldn't pay me any more.
"I said, 'Okay, John, you are a real man. That's fine. Bernard will pay me'. And he head-butted me."
Tomic's Spanish lawyer, Carmen Dieguez, told reporters that while her client did not dispute that he had caused Drouet's injuries, he had no choice but to head-butt him as Drouet had grabbed him by the arms.
Tension between the pair had been rising for some weeks and it was not the first time they had clashed, she added.