A "moonball" in tennis is considered to be one fo the most difficult and off putting styles of play to face. Once mastered, it can be used to throw your opponents off their game and keep the ball in play indefinitely.
To serve a moonball, a player is meant to create as much topspin as possible to give the ball a high, arching angle as it approaches their opponent. The perfect moonball lands almost directly on the opposite player's baseline. This and the high arc of the ball makes it difficult for them to handle the return properly.
A moonball can be an excellent technique to use against power hitters because it can also be used to slow the pace of play. If an athlete relies on their power to win, they can sometimes look to make their matches as short as possible. A mastered moonball is the perfect way to counter this. The deep court position will also prevent a power hitter from getting as much on their stroke as possible.
Patience is the key to defending a player who uses the mooball as a signature move. If you take the time to settle into a match and shed any initial frustration with being set so far back on the court, you will be able to settle into a routine to get into the best position to counter these high altitude serves.