Robert Kubica has reportedly dropped out of the race to complete Williams's 2018 driver lineup.

While the Pole was earlier tipped as the favourite, authoritative and widespread media sources claim the frontrunner is now in fact former Renault reserve driver Sergey Sirotkin.

It was reported this week that Russian Sirotkin has the powerful financial backing of the SMP group behind him, as it offers Williams $15m (£11.2m) for the seat.

Involved in the SMP Racing programme is former F1 driver Mika Salo, who admitted that Williams's decision is now "close".

"It is getting close to the time," the Finn told Ilta Sanomat newspaper.

As for how close that decision is, Salo added: "Pretty close. It is between him and Kubica.

"Both of them did a good job [at the Abu Dhabi test]. Williams now has to take a decision. Let's see which of the two they take."

Polish journalists report that while Kubica did well during his tests for Williams, Sirotkin was ultimately faster. Fellow Russian Daniil Kvyat is now regarded as Williams's 'Plan B' in the event that Sirotkin is not selected.