Arson detectives arrived at the Power Gymnasium on Holmes Street in Brunswick after the blaze started at 5am.
Crews were faced with thick black smoke and ferocious flames billowing from the window and roof.
Thirty firefighters brought the fire under control in half an hour.
"The dangers of going into an uncontrolled environment is you're not really sure whether the fire is going to extend and get you trapped in there but we have good procedures to cover us for that," Fire Rescue Commander Matthew Blandford said.
When crews managed to extinguish the flames, firefighters used breathing apparatus to enter the building but found no one inside.
The fire destroyed about $250,000 worth of property and equipment.
"It's mainly constricted to the front part of the building, however the entire building is damaged by the fire, heat and smoke," Blandford said.
The business is a popular boxing gym, with former Mongols bikie Abdulrahim posting several videos to social media of himself training inside.
Fire Rescue Victoria and Victoria Police are not treating the incident as suspicious but investigators will be on scene for the rest of the day to determine how the blaze started.
