He said, however, that he was "not 100 percent happy" with his day's work, which amounted to two Hungarian GP race distances.
"You can predict it wasn't easy, which I'm not hiding, but it was good...I could drive I think tomorrow easily so the fitness level is good, which is a good sign," Kubica told reporters.
"There are sides which need to be improved or need to get better but it's always like this...once you are here, you want to do better and more. But I have to say we have to be happy and I'm happy."
It was the first time Kubica, a race winner with BMW Sauber in 2008, had driven a 2017 race car and the two-day test was also his first time back in the paddock since his accident.
He had always said he would only return as a driver and not as a guest.
Speculation about Kubica's return has gathered pace since he impressed the team in the 2012 Renault.
"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 had said in July.
The Frenchman added that Renault had "not seen any obvious roadblocks" to a possible comeback.
Renault look likely to have a vacancy at the end of the season, and maybe sooner, with Britain's Jolyon Palmer struggling.
Palmer is one of just two drivers yet to score a point in 11 races this season and has been eclipsed by German team mate Nico Hulkenberg.
Vettel, the race winner on Sunday at the Hungaroring, set the pace on Wednesday with British 17-year-old Lando Norris second on the timesheets with 91 laps for McLaren.
"His feedback with the engineers has been valuable and accurate, and he’s certainly an asset to our test driver line-up -- not to mention a potential star of the future," said Racing Director Eric Boullier.
Italian Luca Ghiotto did the most laps of anyone on track, 161 with Williams, on his test debut for the team.
(Reporting by Alan Baldwin; Editing by John O'Brien and Toby Davis)
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