Why Kubica's F1 racing return was put on hold

It all started with a harmless conversation. Williams was evaluating its driver options for 2018 and Robert Kubica wanted to find out whether he still had the ability to drive a Formula 1 car at a high level, after sustaining life-threatening injuries in a rally crash back in '11.

Renault started the evaluation, then Williams chose to pick up the baton. It and Kubica met to discuss how they could help each other out. There were echoes of 2013, when Felipe Massa and Williams started conversations because their aims aligned.

The difference in this instance was that Williams first needed to evaluate Kubica's physical ability to drive an F1 car, given the severe injury he suffered to his right arm, and then find out if he was the quickest driver available to the team.

Tests in the 2014-spec Williams car were useful - but, as Massa said, they couldn't give the team enough information to judge Kubica's potential in '18 machinery. The cars have changed dramatically since 2014. He needed to test a '17 car. So Williams put him in the FW40 for the post-season finale Pirelli test in Abu Dhabi.

His long-run pace, so far as can be analysed from a tyre test, was solid but not spectacular. His short-run pace, less solid. But together it wasn't enough to rule him out of contention. That is, until his data was put alongside Sergey Sirotkin's.

Sirotkin was a latecomer to the Williams party. The Russian fulfilled reserve driver duties for Renault this season. That team says he is quick and that his technical feedback is strong.

Initially Williams was evaluating him as a potential reserve for 2018, and since it had an opportunity to put him in the car in Abu Dhabi, it did so. He got equal seat time to Kubica, and while the programmes they ran were different, making headline comparisons of lap times irrelevant, Williams would still have enough data to weigh the two options.

On Wednesday night, as the test came to a close, Williams had a problem on its hands. Kubica, the driver it wanted to succeed, was on paper not the best option based on initial analysis.

The team headed back to Grove and delved further into the data. As time went on, it became increasingly clear that Kubica's speed was not a match for Sirotkin's, based on the running in Abu Dhabi.

Kubica's consistency was solid, but the ultimate pace over long runs was not as good as Sirotkin's. But it was the shorter performance runs that really hurt Kubica's chances. He simply wasn't quick enough.

Williams knew that signing Kubica would be a publicity coup, since the team would be responsible for bringing one of F1's greatest talents back to the championship and giving him a second chance. It's a PR dream. But all along, Williams has said it would make a decision based on pure performance.

Still, there was disappointment within Williams when it became clear Kubica's performance on track didn't put him top of the pile. In other areas Kubica led the way. His feedback was impressive and he quickly built a rapport with the engineers. It was abundantly clear he was a driver that would fit right in at Williams.

There were also signs that he could potentially return to his very best, given more time. How much time he needed, though, is another matter, and that uncertainty created an element of risk. Like Renault, Williams was not willing to take that risk.

In Sirotkin, Williams has a 22-year-old, 11 years Kubica's junior, at the start of his career and showing signs of real potential. From an engineering perspective, he was the best bet. But it would mean Williams having the youngest line-up on the grid, which is a downside; a more experienced driver might be better placed to help drive development, and reverse the team's recent drift from competitiveness.

But Sirotkin was the fastest - and that is what the team had elected to base its decision on. It also helps that Sirotkin brings a substantial financial package.

Kubica has come a long way in 12 months, putting himself through a rigorous training regime, and that is credit to his desire, determination and natural ability

Williams will refute that it is becoming a team that needs to take pay drivers. Its argument would be to say what right-minded team would turn down money if it's on offer?

Sirotkin's dowry, which comes from SMP Racing, is around £15million. Initially his budget was around the same as Kubica's at approximately £8million. Paul di Resta and Daniil Kvyat are believed to have had similar support. But in a bid to get the deal over the line, Sirotkin's backers found a way to almost double it. That's some impressive financial clout and Kubica couldn't compete.

Had Kubica shown sufficient speed, though, the size of the budget wouldn't have come into it. Williams would have signed him and taken his more modest offering.

Confirming the legitimacy of Sirotkin's package took some time, which caused a delay in announcing the deal, but ultimately both sides were happy and a deal was struck.

They say time waits for no one, and seven years is a lifetime in F1. The performance landscape has changed dramatically. Returning at a high level, even without Kubica's injuries, would have been a challenge.

Kubica himself admitted that his driving was 90% the same as before his accident. In modern day F1, when the field is so close, that's not good enough. And Kubica, deep down, will know that.

But what he has achieved since he suffered life-threatening injuries in a rally crash in 2011 is nothing short of extraordinary. His physical, mental and racing development in '17 alone was remarkable. It is credit to his desire, determination and natural ability.

Williams believes Kubica is a great asset and there is more to come. It wanted more time to evaluate his potential. Kubica wanted to keep his dream alive.

So talks between Kubica and Williams changed focus and a deal was struck whereby Kubica will be the reserve and development driver.

Crucially, he will get plenty of time in the car, driving in pre-season testing, in-season testing and a number of race weekend practice sessions. He will also provide support to the race drivers while helping drive car performance and development.

And he will support the team's sponsorship programme, which in turn will smooth relations with title sponsor alcohol brand Martini, which requires a driver over 25. Williams needs his expertise, given it has the most inexperienced line-up on the grid.

Kubica needs more time in the car and to be around an F1 team to prove he has what it takes to return to the grid. Kubica says the role is "an important step" in achieving his "ultimate goal" of a racing return to F1.

Achieving the feat is still a long way off - but the dream is not dead yet.