Knocking on the door of a Williams race-seat, Robert Kubica concedes that the final step in his return is the “most difficult one.”

Kubica is believed to be Williams’ preferred candidate to partner Lance Stroll next season after coming through the end of season Abu Dhabi tyre test.

The Polish driver, who injured his right arm in a rally crash in 2011, posted the team’s fastest time at the test using the new Pirelli’s hypersoft tyres.

Williams, though, insisted after the test that they were still evaluating their options.

“When you feel there’s a good chance to be in a position to do properly the job it’s a difficult period,” Kubica told Sky Sports News.

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