The low-set brick and tile home in Holmview, south of Brisbane, was gutted in the blaze about 10:30pm Saturday.
The 19-year-old's carer, who was in the house at the time, managed to drag the injured man through the front window of the property before he got stuck.
Neighbour Matthew Bartholomew came rushing to the scene when he heard screaming in the street.
It was then he saw the young man stuck, and rushed to help get him out.
"He was passed out in the window and we ran over there and we ripped the window open and we dragged him across the street, just while we waited for the paramedics and fieries to come," he said.
"When we got him out, he was already pretty burnt."
But Mr Bartholomew has played down his heroic actions.
"I'm alright. Nothing happened to me. I was lucky," he said.
'It was horrifying'
Next-door neighbour Jarrod Kiely said he was asleep and did not hear anything until someone banged on his door to alert him to the blaze.
"As soon as we got across the road we were confronted with the young bloke laying on the side of the road.
"It was horrifying."
Mr Kiely said his partner, a nurse, came to the man's aid.
It took about a dozen firefighters half an hour to extinguish the fire, which gutted the house.
Paramedics said the 19-year-old has burns to 80 per cent of his body.
He is in a serious condition in Royal Brisbane Hospital.
Fire investigators were at the property on Sunday to determine what caused the fire.