"We've let our best kids play practice sets with the green ball, and every time we've done that the coaches say they're playing a better brand of tennis, " said Patrick McEnroe, the USTA's general manager of player development.

WSJ: Is Tennis Getting Too Soft?

McIlroy was out in a three-ball with team-mates Llewellyn Matthews and Jamie Moul in Tuesday morning's practice at RCD.

BBC: McIlroy magnetism