Polish driver is in contention with Russian Sirotkin to fill the vacancy, seven years after his last race in Formula One.
Kubica made to wait on F1 return as Williams delay announcing new driver until January
Robert Kubica will have to wait until the New Year to find out if Williams will offer him a route back to Formula One, although Russian Sergey Sirotkin now appears to be the favourite to partner Lance Stroll next season.
Kubica is aiming to make a remarkable return to F1 seven years after he left the sport following a rally crash that left him with a partially severed forearm.
The Polish driver had been testing for Williams in Abu Dhabi following the conclusion of the 2017 season, and it appeared he would be chosen to fill the seat vacated by the retired Felipe Massa.
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However, Williams have said any decision has now been put off until January, with the team insisting any decision would be based on performance while giving few hints about who is likely to succeed Massa.
Sirotkin, a 22-year-old development driver for the Renault team this year, has been backed by Russian billionaire Boris Rotenberg and his SMP Racing.
Rotenberg's brother Arkady is a close ally and former judo partner of President Vladimir Putin.
Leaving aside his significant backing, the Russian impressed in testing in Abu Dhabi after he was passed over by Renault when they were looking to replace Britain's Jolyon Palmer.
Both he and Kubica, 33, were at the factory last week for debriefs and further assessment, with media reports suggesting Daniil Kvyat was also in the frame after being dropped by Red Bull Racing and Toro Rosso.
Kubica was tipped as a future champion before a 2011 rally accident partially severed his right arm, and while there are some lingering doubts about his ability to race without full movement the Pole has said he has no problems and is fitter than ever.
Kubica has tested twice for Williams this year, as well as with Renault, and his case for the seat is being argued by former world champion Nico Rosberg, who is acting as a go-between.
Williams' head of performance engineering Rob Smedley said last month he hoped a decision on the driver would be made by Christmas but that the team was in no hurry.
"There’s no reason for us to rush into it," he said then. "We are the best remaining seat so we have that advantage in our armoury. We can wait and we can evaluate every little bit and do all the due diligence necessary."