There is no such a thing as "for a tennis player". Tennis is not like basketball. If you're short/average/tall for the wider world, then you're the same for tennis.
Having said that, Nishikori is someone that can generally be regarded as a "just below average" with his 1.78m hight. Certainly not a real shorty short person that justifies the dismissive sneer he got at the hands of Sky sports commentators (and the English have no right to mock someone's height anyway as I can personally attest they are on average the shortest people on this continent outside some southern/Mediterranean regions
What on earth are you talking about?
The average heights are as follows:
top 10: 187cm
top 50: 187cm
There's only one shorter player in the top 50 and that's Ferrer. The fact is, he is indeed short for a professional tennis player and I'd class him as below average in every day life.