All right
Hewitt is Potato because 1) his head looks like a potato, LOL and 2) during the 1999 Davis Cup tie between Russia and Australia on grass, Kafelnikov called the courts "potato fields" meaning they were so pockmarked and uneven and basically really shitty.
Kafelnikov's first name is Yevgeny, and "Zhenya" is a dimunative name of "Yevgeny" (aka a nickname).