The names "Prussia" and "Russia" are not related. That is a misinterpretation that, unfortunately, still lingers in our modern books. These two names are similar only in English. If you dig deeper, you will find the root of Russia in the name Rus, which refers to the Vikings who settled in present day Russia. Prussia in its original is Prusa, which became Preussen in German, and was derived from one specific band of the Prusa peoples.
Also, the Prusa people were not wholly extinguished by the Teutonic Knights. There were 2 small pockets remaining, both on the Courland Spit, until 1945. It took the German/Russian rivalry to finish off what the Teutonic Knights had begun. There is an excellent book on this topic called "Teuton and Slav" by Hermann Schreiber (it has been translated into English).
There is a bounty of information on linguistics compiled by Hermann Frischbiers if you can read German.
Other excellent sources, in English, are "Before the Storm" by Marion Countess Doenhoff, and the published diaries of Agate Nesaule, Modris Eksteins, Marie Vassiltchikov, Irene Zarina White, Evelyne Tannehill, Christa Eckert Blum, Margarete Mueller, and Gunter Nitsch.