* Sirotkin joins Stroll at Williams
* Russian rookie replaces Massa
* Announcement completes the 2018 starting grid
* Kubica to be team reserve driver (Adds details, Kubica joins as reserve)
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 on Tuesday in what the Formula One team said was a multi-year deal.
Kubica, who had been hoping to return to the starting grid for the first time since a 2011 rally accident partly severed his right arm, was appointed reserve and development driver instead.
Williams made the long-awaited announcements in separate statements, with 22-year-old Sirotkin filling the last remaining vacancy for the season that starts in Australia on March 25.
The Russian will partner 19-year-old Canadian Lance Stroll, the billionaire's son who made his debut with the former world champions last year, in a lineup that will be the youngest on track this 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, whose oligarch brother Arkady is a former judo sparring partner of President Vladimir Putin, on his way through the junior series.
The team did not give any financial details of the deal.
Deputy team principal Claire Williams said the Mercedes-powered team, fifth overall last year, 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 at one point looked the favourite for the drive, with 2016 champion and former Williams driver Nico Rosberg championing his cause.
Williams said the Pole will take part in testing before and during the season as well as some Friday practice sessions and simulator work.
"It's a great pleasure to be back in Formula One," said Kubica. "My ultimate goal remains to race again in Formula One and this is another important step in that direction."
Williams technical head Paddy Lowe described the Pole, a winner in Canada with BMW-Sauber in 2008, as a legend of the sport.
"To overcome his injuries, return to fitness, and drive a Formula One car again is a remarkable achievement, and one that few considered possible," added Lowe.