I don't know who the "experts" are that the OP says are saying you "have" to be 6'4" to play quality tennis -- I do know that some of these same alleged experts do seem to be decrying how hard movement gets when a player gets to be about 6'5" or so.
That said, someone on one of the other threads the other day was decrying players who were "short" as "poor representatives of the game". Do people not know about Ollie Rochus?
What is Kei, 5'9" or so? He's not short.
He's always been good enough that Rafa said some years ago that he would probably be a Top Ten player some day. I really am not sure I understand all the surprise.
"Before and after the match you are the same, no?" - RN
"Pain is only temporary. Victory is forever." - RF