(CNN) -- They are tall, powerful and capable of serving a tennis ball at over 150 mph (241 kph). They are the rocket men of tennis, and they're getting faster.
Leading the race is Ivo Karlovic, the 6 ft 10 in Croatian they call the "King of Aces," who was clocked at 156 mph (251 kph) during a Davis Cup tie in Zagreb last month.
The 32-year-old's world record is unlikely to go unchallenged for long. Andy Roddick has twice hit the 155 mph (249 kph) mark, and 20-year-old Canadian Milos Raonic is already firing at 153 mph (246 kph).
But as serves continue to get faster, tennis is staring down a difficult problem -- what to do when rallies become a rarity, and every other point is an ace.
"Players like Raonic and Karlovic combine great technique with height and long arms to generate racket speed," said former Wimbledon champion Pat Cash, of CNN's Open Court.
"It seems the only way to slow the serves down is to limit the size of players, cut some of their arms off, or make them stand in a hole."
But there may be an alternative. In 2002, the International Tennis Federation (ITF) included a new, larger tennis ball in its official rules.
While its primary purpose is for recreational players and use at high altitude, the Type 3 ball is also intended to "help curb where necessary the dominance of the serve and corresponding lack of rallies in the professional game."
The graphic above demonstrates how. The 6% increase in diameter means the ball travels more slowly due to greater drag, and also bounces higher. Put simply, it makes the game easier.
"That's why we've got them. We're waiting in the wings for a time when there are too many aces in professional tournaments," said Jamie Capel-Davies, Senior Project Technologist at the ITF.
"For the moment our research suggests reaction times are keeping up with the increases in serve speed, because the number of aces has not really increased since 2002. But when we see evidence that aces are going up, that's when the bigger balls will come into play."
ITF research suggests it's a matter of when, and not if. The mean average of men's serving speed in the four grand slams rose from 130 mph (209 kph) to 137 mph (221 kph), between 2002 and 2010.
The world record stood at Greg Rusedski's mark of 149 mph (240 kph) in 2001. Just 10 years later, and the 160 mph (257 kph) barrier is very much within reach.
Bigger, stronger players, along with advancements in racket technology, are pushing the envelope. But if the rocket men keep serving faster, they might not like what's coming next.
"The U.S. seniors used those bigger balls for a couple of years, and they were soundly voted down. Not one player like them," Cash said. "It was impossible to hit a winning volley and the ball dipped severely -- making it easier to keep the ball in court.
"Introducing them would be a move in the wrong direction. Tennis is becoming like a video game as it is, with the lack of variety on the shot-making front. Taking the volley out of the game with larger balls will ruin the game. Bring your caffeine pills to keep you awake."
Capel-Davies admits the bigger balls won't be to everyone's liking: "Players will feel like they have to hit the ball harder to gain the same power with their shots, and some of our research suggests they could tire more easily."
When the alternative could be a sport where the giant serve is all-powerful, and matches could effectively be over in half the time, the ITF may be left with little choice.
Reaction times have a ceiling, and when that's reached a glut of aces will likely prompt the ITF to act.
Forget "New balls, please" -- the call for the next generation could well be, "Bigger balls, please."