I've wondered as well. I suspect the biggest area of an early coach's impact is on the serve. Even Fed has said it is "about your first coach" and hard to change afterwards.
As for the 10-15% difference made by the coach, at first blush that sounds small. But given the fine line between winning and losing atop the ATP, 10% could be huge!
As for the 10-15% difference made by the coach, at first blush that sounds small. But given the fine line between winning and losing atop the ATP, 10% could be huge!

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