A serve that the receiver fails to touch is called an ace. It is a legal serve that lands in the correct service box and wins the point immediately because the receiver cannot make contact with it.
An ace can be hit through sheer speed, but placement and disguise are often just as important. Servers target the corners, the center line, or the receiver’s body, sometimes using slice or kick spin to make the ball difficult to reach. A serve that is touched but cannot be returned is usually recorded as a service winner rather than an ace.
An ace is not the same as a double fault. A double fault gives the receiver the point after two unsuccessful service attempts. A let, by contrast, occurs when a serve clips the net cord and still lands correctly; the serve is replayed under standard professional rules.
Ace totals are commonly used to compare serving performances. Ivo Karlović, John Isner, Serena Williams, and Goran Ivanišević are among the notable big servers associated with high ace counts, though match conditions and court speed also matter.