Roberts was widely praised after he, FIA medical car driver Alan van der Merwe and local marshals rushed to Grosjean's aid after the Frenchman's huge crash on the first lap of the race.
The Frenchman escaped with burns to his hands but will spend the night in hospital.
"There was a massive flame and we arrived to a very odd scene where you've got half a car pointing in the wrong direction and just across the barrier a mass of heat," Roberts told Sky F1.
"Then looking to the right at that point, I could see Romain trying to get up.
"We needed some way of getting to him, so we got the marshal there with the extinguisher, and the extinguisher was just enough to push the flame away as Romain got high enough, so I could reach over and pull him over the barrier."
Roberts led Grosjean to the medical and gave him a quick check over.
"I think I told him to sit down. He was obviously very shaky, and his visor was completely opaque, and in fact melted. I managed to get his helmet off to check everything else was OK.
"It was going to be flames, smoke inhalation, airway issues, and that nothing went up to his helmet, and we had a look at the helmet as well.
"But looking at him clinically we were quite happy with him from a life-threatening injury point of view, so it was about making him comfortable from the injuries we could see.
"He'd got some pain in his foot and hands, so from that point we knew it was safe enough to move him around into the car for protection and get some gel on to his burns, and then into the ambulance and to the medical centre."
Medical car driver Alan van der Merwe stressed the incident showed how practice paid off.
"A lot of it is down to preparation," he said. "When you get to something like this, and we've not seen this combination before.
"I've not seen fire like this in my stint as the medical car driver, and a lot of it new and unknown territory, so we can only be as prepared as our own ideas.
"We do a lot of checklists and a lot of preparation, talking about scenarios, but this was crazy.
"Honestly, to get there and to see half of the car and the other half nowhere to be seen and just a huge ball of flames so you have literally seconds, thinking on your feet, so preparation only gets you so far. Then it is down to instincts and quick thinking."