High and deep: depends what you mean with DIY tennis coaching
-Tactical, shot selection, strategy?
-Mental toughness, control under pressure, concentration, winning attitudes
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As to the question of parents coaching kids successfully I think we can take out Sharapova (as her dad brought her at 7 or something to Bollettieri and a bit later Robert Landsdorp), but put in C Wozniackis dad, an ex football player who trained her up to 10 or something . Also Tomic dad I think was an amateur coach.