Thanks MCLOVIN, you correclty pointed out that I was wrong, but your answer also is incorrect. A let is only called if it is UNCLEAR whether the correct ball was played. If the returner hits the wrong ball as a winner, but it is clear later that he played the wrong ball, then no let, he loses the point. The rule is here:
"Case 2: A ball in play hits another ball which is lying in the correct court.
What is the correct decision?
Decision: Play continues. However, if it is not clear that the actual ball in
play has been returned, a let should be called."
For instance, if the ball being served is a wilson 3, but the ball that was returned is a Penn 1, then it is clear he returned the wrong ball, and he loses the point in that weird case.
Originally Posted by McLovin
Your 1st serve was a fault, but the opponent did not clear the ball from the court and it is sitting inside the lines of play. During the point, your shot hits the ball on the court, and both balls go flying into the air. Your opponent unknowingly plays the wrong ball & hits a winner. Who's point is it?
The answer is: You play a let (see rule 25): http://assets.usta.com/assets/1/USTA...c_13_16051.pdf
Additionally, if it is clear
the opponent hit the correct ball, they win the point.