According to Guinness World Records, the fastest serve of a tennis ball was recorded by former Australian professional Samuel Groth during a second-round match at the Busan Open Challenger Tennis in Busan, South Korea on May 9, 2012. Against Belarusian Uladzimir Ignatik, Groth sent down an ace clocked at 163.4 mph, thereby obliterating the previous record, 156 mph, set by Croatian Ivo Karlovic during a doubles match against Germany in the first round of the Davis Cup World Group in Zagreb on March 5, 2011. For the record, Groth also registered serves measured at 158.9 mph and 157.5 mph, but still lost the match.
Serving speed is measured, immediately after the ball has left the racket, by speed guns positioned behind the baseline at either end of the court. Each gun emits a stream of radar pulses, some of which are reflected by the ball back to the gun, where the difference between their frequency and those of the original pulses is translated into the speed of the ball, in miles per hour (mph). Not all speed guns are created equal, so the Association of Tennis Professionals (ATP) does not formally recognise speeds recorded at Challenger Tour events.
Thus, as far as the ATP is concerned, the fastest serve of a tennis ball, 157.2 mph, was recorded by American John Isner against Australian Bernard Tomic during a singles match in the quarter-finals of the Davis Cup World Group in Melbourne on March 6, 2016. Isner also has the distinction of serving the most aces in the history of the ATP Tour, 14,177 at the time of writing, and having played in, and won, the longest tennis match of all time; after 11 hours and 5 minutes, he finally defeated Frenchman Nicolas Mahut 70-68 in the final set of their first-round match in the men’s singles at Wimbledon in 2010.