The oldest man to reach the third round in 25 years, Karlovic believes it will difficult for Murray to return in his serve in their upcoming clash.
Andy Murray's French Open has already been a hair-raising ride and things are unlikely to get much simpler on Friday as Ivo Karlovic trains his mighty serve on the world number two.
Karlovic, 37, may be the oldest man to reach the third round of a grand slam for 25 years, but his thunderbolt delivery is still a match for anyone, even one of the game's great returners, the Croat insists.
"Andy is a great returner but I believe in my serve and when it's on there is no one who can return it," 27th seed Karlovic, the all-time ace leader on the ATP Tour with 10,696 and counting, told Reuters on Thursday.
"I've only lost my serve twice so far here so I'll focus on that and try to take my chances when they come."
Karlovic won a fifth-set decider 12-10 against Australian Jordan Thompson on Wednesday, a match that lasted more than four hours.
There was no rest for the 6ft 11in Croat the following day though as he was in doubles action, before focussing on how to improve a 0-6 record against Murray.
He has taken heart from fellow 37-year-old Radek Stepanek's epic struggle against Murray in round one.
"Radek played a great match and mixed it up really well," Karlovic said. "I have a similar kind of game too so I was making notes and I will try to use those on Friday."
Karlovic and his serve first made an impact when he stunned defending champion Lleyton Hewitt in the first round of Wimbledon in 2003.Thirteen years later the serve still works like clockwork.
The ATP's statistics show he won 95 percent of his service games in the last year, more than any other player.
He has other attributes though, a sound volley, under-estimated groundstrokes and super fitness levels, as proved when out-lasting 22-year-old Thompson.
"It was not just about experience that win," he said. "It was about physical endurance too. I work all the time on strength conditioning, sprinting, making myself a better athlete. It was excruciating against Jordan but I felt strong."
While Karlovic's game has evolved from those early days, his serve will be the key when he faces Murray, who he has pushed hard in the past, notably at Wimbledon last year.
Their six prior meetings have produced eight tiebreaks and it would be no surprise if they contested a few more on Friday.
"It's an open match," said Karlovic, who honed his serve belting thousands of balls alone while growing up in war-torn Croatia.
"He's already had two five-set matches so I don't think he'll be at his 100 percent.
"Difficult match for sure. But I'm ready to go five if I have to. I believe I'll have the opportunities to win and I'll just have to make sure that I take them."
Karlovic has no intention of retiring any time soon, especially with a howitzer delivery still feared on the Tour.
"It's easier when you hit an ace and you take your opponent out of the picture," he said. "Everyone would want to have a serve like that if they could.
"I'm still improving as a player. Four years ago If you asked me if I would still be seeded at Roland Garros I would have thought it was unbelievable.
"I want to do this for many more years because tennis gave me everything in my life."
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 "Ivo Karlovic")
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