Kubica, now 32, insists it was not a publicity stunt.
“The approach by Renault was a nice surprise,” he is quoted by Italy’s Autosprint.
“They tried to keep it as secret as possible, as the test for them was not about putting me in the car and making a good story, even if it generated a lot of interest.”
Earlier, Kubica had almost completely ruled out returning to F1 due to permanent physical movement limitations, but he said the test showed him that he can drive a competitive full race distance.
As for that “comeback” word, the former BMW and Renault driver said: “It’s not for me to decide what the future holds.
“It’s nice to know that I can do what I did 6 years ago in spite of my limitations.
“I feel ready, I have worked so hard and I can say that I have never trained so hard and well even in the years I was racing.
“I am looking forward, I hope there is something else but it’s not for me to decide,” Kubica added.
He said the years since leaving F1 were “difficult”, but “Last year I said I wanted to go back to formula one”.
“I just did one test, but for me it’s an important goal and I feel I am starting a new path even if I don’t know where it will take me.
“But I do not hide that I would like to go down it, because I had good feelings. It doesn’t depend on me, but I do know I can give my best,” added Kubica.