How fast was the fastest recorded tennis serve, in miles per hour?

How to get close to this number

The reaction time of the returner sets the ceiling. A serve has to cross the court in roughly the time it takes a human to react at all, which the explanation puts at about half a second. Work backwards from the length of a tennis court and that half-second, and you land close to the answer without knowing any record at all.

The instinct to compare with a baseball pitch or a cricket delivery leads people astray in both directions. A tennis ball is lighter and is struck with a long lever rather than thrown, so it leaves faster than any thrown ball — but it also decelerates faster through the air, which is why the speed at the racket and the speed at the baseline are very different numbers.

This particular record comes with an asterisk, and the honest thing is to say so. As the explanation notes, it was set at a lower-tier event and is not universally recognised; the fastest at a Grand Slam is lower. Radar equipment, its calibration and the point in the ball's flight where it measures all vary between tournaments, so serve speeds are less comparable across events than the leaderboards imply.