Modra suffered head injuries when a tree branch broke through the windshield of the truck he was driving near his cattle property, south of Adelaide, on Thursday.
The former Adelaide and Fremantle star was in a southern Adelaide hospital in a critical, but stable condition on Friday afternoon.
As the AFL and wider communities expressed shock at the accident involving a man described as a sporting icon, one of the first people to attend to Modra told of the accident scene.
Anthony, who did not want to give his surname to protect his anonymity, and his partner Sarah, were driving along Range Road at Back Valley, about 90km south of Adelaide, about 5.15pm (local time) on Thursday.
"I came around the corner and I saw a truck on the opposite side of the road so I started braking," Anthony told AAP.
"He was driving towards Waitpinga and a tree branch fell and hit his windshield and he managed to control it (the truck).
"Then, he got out the cab himself and went to the side of the road and fell down.
"It was very windy. It wasn't raining but the roads were wet, but that had nothing to do with the accident itself. It was just a tree branch falling that caused the accident."
Anthony and Sarah are both trained as first responders, but were not employed first responders when they came across the scene.
"I just saw a man in trouble and knew what I had to do," Anthony said.
"I don't follow sport ... it wasn't until this morning when I checked online for any information on it that I found who he actually was.
"He's a man that needed help, that's all it is to me."
Modra's wife Erica had earlier thanked Anthony and Sarah via the footballer's former Crows captain and close friend Mark Ricciuto.
Ricciuto relayed messages from Erica on his Triple M breakfast show on Friday morning.
"She just wants to say a very big thank you to the first responders, called Sarah and Anthony, who saved his life and helped talk through to Erica while things were going down," Ricciuto said.
"She said he's going all right. It's pretty amazing that he's got through it."
Modra, 57, was renowned as one of the game's greatest drawcards for his spectacular high marking and prolific goal-kicking.
He played for the Crows between 1992 and 1998 and then spent three years at the Dockers until retiring.
"In his time in the game, it was nearly like pop star status," said current North Melbourne coach Alastair Clarkson, who played against Modra.
Adelaide coach Matthew Nicks said he had never witnessed anyone with the same popularity as Modra, who remains involved with the Crows.
"There's certain people that have an aura about them and Tony is one of those," Nicks said.
"Our prayers are there, our thoughts, everything we can possibly do - which is not much at the moment - is there for him. We just hope he fights through this one."
SA Premier Peter Malinauskas and the state's opposition leader Ashton Hurn both described Modra as an "icon" of the state, while Port Adelaide coach Josh Carr described news of Modra's accident as "devastating".
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