Returning Formula 1 driver Robert Kubica believes he is only "20 per cent" ready for the first race of the 2019 season that gets underway later this month.
Kubica hasn't raced in Formula 1 since 2011 following a serious injury in a rally-car that same year.
Williams offered the Pole a place back on the grid to once again compete, with the experienced driver joining the inexperienced George Russell at last season's bottom team.
The British side suffered a major setback at the start of testing - with Williams unable to get their car on track for the opening two days of pre-season testing in Barcelona, something that led to massive criticism for the former world champions.
"That's the situation, I cannot change it," Kubica said to Motorsport.com. "I have to make sure we are doing everything what we can, from my side. Coming back after eight years, probably I know 20 per cent of the things I should know before going to Australia.
"The rest is unknown. I haven't done longer than a 15-lap long-run. There is a lot of question marks. But that is the reality, we cannot change it."
Kubica admitted his frustrations on the final day of testing, with Williams possessing a lack of spare parts which ultimately held them back on day eight in Barcelona.
"My last real run when I feel the car well was (Wednesday) afternoon, first run, when the car did things which I was surprised (by), a really nice surprise.
"So I get up really a lot of confidence, but since then my confidence disappeared, because the car was not in the right state."