The spokesman, on his way to this week's Belgian Grand Prix at Spa-Francorchamps, said Sirotkin had been at the Hinwil factory on Tuesday and Wednesday for a seat fitting and to talk to engineers.
The Sochi event has been pencilled in for September 27 in the Black Sea resort that is due to host Russia's first grand prix next year, providing the local authorities meet all the conditions.
Sirotkin, who would be by far the youngest F1 driver if he does race next season, currently lacks a superlicence to compete in Formula One or take part in Friday practice and testing is banned during the season.
The demonstration run - likely to be little more than driving up and down in a straight line in an older Sauber - will be a first step towards the ultimate goal of getting him on to the starting grid.
Sauber said last week that the collaboration with the Russian partners was progressing well despite "erroneous reports and rumours circulating in certain media".
A statement on the team website (www.sauberf1team.com) said the contract with Sirotkin was in place and initial payments to the team had been made by their Russian partners.
(Reporting by Alan Baldwin in London, editing by Ken Ferris)
