( 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 wo n'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 . ''
Ivo Karlovic broke the world record for the fastest serve in a Davis Cup match in March . The Croat was clocked at 155 mph ( 251 kph ) to beat Andy Roddick 's previous mark . TL;DR They are seen as a potential solution if the number of aces begins to increase .