By Alan Baldwin
LONDON, Jan 16 (Reuters) - Russian rookie Sergey Sirotkin will race for Williams this season after being chosen ahead of Polish rival Robert Kubica in a multi-year deal.
The Formula One team made the long-awaited announcement with a statement on Tuesday, filling the last remaining vacancy on the starting grid for the season starting in Australia on March 25.
The 22-year-old will partner 19-year-old Canadian Lance Stroll, who made his debut with the former world champions last year.
Sirotkin replaces Brazilian veteran Felipe Massa, who retired at the end of last year.
"It took a huge amount of work to get where I am, and I'm really happy and thankful to everyone involved," said Sirotkin, who was previously on Renault (PA:RENA)'s books as a development driver.
"The result of our combined efforts has helped me achieve my dream, and rest assured the team can rely on me to deliver my best."
Sirotkin has been backed by Russian billionaire Boris Rotenberg on his way to the top.
Deputy team principal Claire Williams said the team had taken their time to evaluate all options but were confident the new pairing would deliver.
"The Williams philosophy has always been to promote and develop young talent and Sergey fits right into that ethos," she said.
"Lance has had a record-breaking debut season, and with a year now under his belt, he will be ready to hit the ground running in 2018."
Kubica, 33, had been hoping to return to a Formula One race seat for the first time since a 2011 rally accident partially severed his right arm.
He and Sirotkin both tested with Williams after last year's season-ending Abu Dhabi Grand Prix.
Kubica will be the team's test and development driver.