The 40-year-old was initially left out of the Team GB squad for Rio but eventually forced her way in and will compete with Victoria Thornley in the double sculls final.
Grainger and Thornley surged through their semi-final but face a battle to finish on the podium in a strong field.
Peter Lambert, Sam Townsend, Angus Groom and Jack Beaumont go in the men’s quadruple sculls final.
There are also British hopes in the men’s double sculls with the partnership of Jonathan Walton and John Collins and in the men’s pair with Stewart Innes and Alan Sinclair competing.
