I think hard work can take someone a long way in tennis, especially if the kid is above average physically. How physically gifted was Jim Courier, or David Ferrer, or Mardy Fish, or Juan Carlos Ferrero, or Carlos Moya.....?
In a post-match Olympic interview, Murray commented on Mo Farah's last 400 meters in the 10,000 meter final (53 seconds), and said that he himself can run 57 seconds for 400 meters flat out. This is not great speed. Now, if he trained specifically for 400 meters he would run faster, but it shows that his base speed isn't overly impressive. A lot of hard work has gone into him being a great mover on the court.