Steven Johnson of Star Towing was seriously injured and sent to St. Vincent Hospital on Saturday.
Monroe County Sheriff's Dale Malone said the hit-and-run crash happened on South Otter Creek road around 8:30 p.m.
Johnson was helping refuel a stranded motorist's car in the northbound lane when a passing by semi struck the side of the tow truck.
The semi scraped the side of it, damaging the side view mirror and striking Johnson who was standing beside the stranded red car.
Johnson has been reported in fair condition.
Anyone with information is asked to call Monroe County Sheriff's Office Traffic Division at 734-240-7560.