Here is the link to the official USTA tennis rules: http://www.usta.com/AboutUs/~/media/USTA/Document%20Assets/2007/02/09/doc_13_16051.ashx
Under Section 24. Player Loses Point (page 19),
The point is lost if:
The ball in play touches the player or anything that the player is
wearing or carrying, except the racket;
This rule does not specify whether the player is inside or outside of the court. This means that it doesn't matter if the player is inside or outside of the court. As long as the player touches the ball with his body or clothing, s/he loses the point.
This interpretation can be verified at the following site: http://www.collinscompany.com/Tennis_Court_Equipment_And_Supplies/Tennis_Rule_20.asp
Under section g., there is a comment on this rule, which states:
USTA Comment: This loss of point occurs regardless of whether the tennis player is inside or outside the bounds of his tennis court when the tennis ball touches him.
Writer's comment: Even though the ball is clearly out if the player was hit by the ball when s/he was standing outside of the court, the player whom was hit by the ball still loses the point because s/he touches the ball with his body or clothing, which ends the point.