Robert Kubica began a Formula One test in Hungary on Wednesday that could determine whether the Pole makes a remarkable race comeback more than six years after a rally crash cut short his Grand Prix career.
The 32-year-old, who partially severed his right forearm in that horrific 2011 accident, left the pit lane to start testing Renault's current car at the Hungaroring circuit outside Budapest.
It was the first time Kubica, a race winner with BMW Sauber in 2008, had driven a 2017 race car after conducting tests in older machines.
On Tuesday, the Pole had made his first appearance in the paddock since 2011, having always said he would only return as a driver and not as a guest.
He had passed the mandatory extraction test, showing that he can get out of the car inside five seconds and replace the steering wheel within 10 seconds, the day before.
"I would like to have a comfortable feeling in the car but also see the team happy with the job I have done," he had said before the test.
"It's not about lap times and pace, but the commitment and feedback I can provide."
Speculation about Kubica's return has gathered pace since he impressed the team in the 2012 Renault car, with its V8 engine and demonstration tyres.
"He is still quick, he is still very consistent and more importantly he has this energy and this drive, this enthusiasm that he has always had," the team's managing director Cyril Abiteboul said in July.
The Frenchman added that Renault had "not seen any obvious roadblocks" to a possible comeback.
Renault are likely to have a vacancy at the end of the season, and maybe even sooner, if Britain's Jolyon Palmer continues his run of poor form.
Palmer is one of just two drivers on the grid yet to score a point in 11 races this season and has been eclipsed by German team mate Nico Hulkenberg.
Kubica has plenty of fans, with a strong Polish crowd attending the test, and admirers within the sport at the highest level.
"Robert's one of the quickest drivers I've ever raced against," Mercedes' triple world champion Lewis Hamilton told reporters at the weekend.
"Just raw, natural talent... not a lot of great, great drivers come through. You have some that are much better than the rest, but still not the greatest. And then you have real special drivers like him."
Reporting by Alan Baldwin.