Durham police said the 42-year-old driver was on his route when he saw the blaze in a garage at the northeast corner of Kingston Road and George Jones Street around 1:30 a.m.
The driver checked the burning building, then went to a nearby building on the property that houses a professional office and a downstairs apartment, said police spokesman Dave Selby.
The driver alerted residents inside, ascertained there were no people in the burning building, and called 911.
Police continue to investigate the fire. Anyone with information is asked to call 905-579-1520.