The PGA superstar says he injured his back Wednesday falling down stairs at the house he's staying at near Augusta ... but tried to soldier through the pain Thursday morning.

Johnson is the #1 ranked player in the world and was the heavy favorite to win the tournament.

Johnson took some warm-up shots at the driving range -- but cut it short. He was seen holding his back in pain afterwards.

He even went to the 1st hole and tried to go -- but decided to throw in the towel.