MOTOR SPORT NEWS - The team are finalising a contract with the 22-year-old, who established his credentials in a test in Abu Dhabi after the end of last season.
The Russian, who has a reported budget of €15m (£13.3m), was faster than rival Robert Kubica after the team had fully analysed all the data from the test.
Sirotkin and Lance Stroll would be the least experienced line-up on the grid.
Sirotkin will be making his F1 debut as partner to a 19-year-old who has raced for only one season.
Williams had been intending to sign Kubica for a return to F1 seven years after the Pole suffered life-changing injuries in a rally crash in February 2011.
But the team's interest in the 32-year-old cooled after he tested their 2017 car in Abu Dhabi alongside Sirotkin.
Sirotkin has a bigger budget than Kubica by a factor of about two, but the team insist the decision is primarily performance-based.
Although there is understood to be no impediment to the Sirotkin-Williams deal, no announcement is expected for at least the next 10 days as the team take their time to complete the details of the contract.
Meanwhile, Canadian Nicolas Latifi has joined Force India as test and reserve driver for the 2018 Formula 1 season.
