Sirotkin, who was a test driver for Sauber in 2014 and Renault in 2016 and 2017, will replace Brazilian Felipe Massa. After Massa retired at the end of last season, Kubica had hoped to get the seat, but instead has been named by Williams as its reserve and development driver. The 33-year-old Polish driver, who last raced F1 in 2010, still hopes to find a way back into the sport that once promised him a glittering career.
"My ultimate goal remains to race again in Formula One and this is another important step in that direction," Kubica said. "It has taken a lot of work to get to where I am now, so I'd like to thank Williams for the opportunities they have given me so far."
Kubica earned 12 F1 podium finishes from 2006-10 with his quick, flashy driving and was seen as one of the brightest talents in the sport. He won the Canadian GP driving for BMW Sauber in 2008, after a second-place finish at the Monaco GP in the previous race.
Four-time F1 champion Lewis Hamilton has said he believes Kubica would have been in contention for an F1 title if not for his gruesome rally accident in 2011. It left him needing seven hours of surgery on a partially severed right hand, and numerous subsequent operations.
Kubica did F1 testing for Renault and Williams team last season, raising anticipation of an F1 comeback. For now, largely because he still has restricted movement in his right arm, he has fallen a little short.
But Paddy Lowe, the chief technical officer at Williams, expects him to nonetheless make an impact in a secondary role. "We anticipate Robert will make a strong technical contribution to the team, using his wealth of experience in track testing, simulator work, and support to the race drivers and engineers," Lowe said. "He is a driver I have admired for many years and I am personally very happy to be working with him."
Sirotkin, meanwhile, is set to become the third Russian to compete in an F1 race after Vitaly Petrov and Daniil Kvyat — who has lost his Toro Rosso seat for 2018. "It took a huge amount of work to get where I am," Sirotkin said. "Rest assured the team can rely on me to deliver my best."
He has been supported through his career by the SMP Racing project of billionaire Boris Rotenberg, a close ally and former judo partner of President Vladimir Putin. Stroll, the son of Canadian billionaire investor Lawrence Stroll, made his F1 debut last year and became the first Canadian on the grid since 1997 world champion Jacques Villeneuve in 2006.
The new season gets under way in Melbourne, Australia on March 25.