Russia's Su-35: Really Just an Su-27 on Steroids or Something More?
The Su-35 is just a heavily upgraded derivative of the Su-27 aircraft (NATO reporting name "Flanker") that was originally intended for export. However, it has been in service with the Russian Air Force since 2014 and made its first combat deployment in Syria in 2015 – where it was employed to provide cover for other Russian aircraft on bombing missions.
Source: nationalinterest.org