The 32-year-old Pole, whose F1 career was interrupted following a rallying crash, finished fourth fastest and completed 142 laps on day two of the post-Hungarian Grand Prix test.

Renault had given Kubica two tests in 2012-spec machinery earlier this year - at Valencia and Paul Ricard - and then decided to run him in the official F1 test to better understand if he is capable of a return in 2018.

One-time F1 race winner Kubica admitted he was "not 100 percent happy" with his return but felt good physically after the eight-hour test, held in hot and humid conditions, and did not feel any pain.

"I would like to have other opportunities but the reality is that I don't know and we'll have to wait and see." said Kubica.

"If we see where I was four months ago compared to where I am now, it is a big change and it happened very quickly.

"So I think if in three months I did improve a lot and moved forward quite a lot, everything can happen in the future. But we have to be realistic, nothing will be easy.

"My target is to get this kind of role in F1 if I can and if I will have a chance but I don't know. One thing is sure, if it doesn't happen, I will not be disappointed because I am looking at this situation very realistically."

Kubica completed 20 stints during the test, including a handful of ultrasoft tyre runs, as he worked down to a best lap of 1.448s adrift of Ferrari pace-setter Sebastian Vettel.

The Pole feels he needs more time in the car to feel comfortable.

"If I get the chance to jump again in the car I will find it much easier, more familiar and it will come more naturally," he said. "For a driver the best feeling you can have is everything comes so easily you don't think about it.

"But when everything if new you have to think about it and concentrate on things, keep analysing why you are driving. Once I jumped out of the car [after four hours of running], I had a big break and I jump back at 2pm and I felt like a different driver.

"The car felt much more familiar and it's a good sign. A lot of people think you improve only by driving but you can improve also by thinking and thinking where you can

improve.

"This is what happened and if I was driving again the next day I would have some idea where I can improve and everything will come easier and more natural which is the target."