ANDY Murray has an appointment with Dr Ivo.

The big-serving 6ft 11in Croatian is not the kind of specialist anyone would want to see, particularly when they are suffering from a sore shoulder. The 36-year-old from Zagreb, the tallest man in the history of the ATP tour, has prescribed the World No 3 some further pain second on Centre Court this afternoon and the Scot will have to be patient and put on a returning clinic of his own if he is to prevail.

Karlovic's top service speed is only 137mph, fully 10 miles per hour slower than Sam Groth's effort against Roger Federer on Saturday. But it scuds down at you from the clouds, finding parts which others cannot reach. Moreover, so far this tournament, he is getting 70% of them in play. This compares favourably with the Scot's 63% and has helped him accrue 136 aces through three rounds.

Andy Murray wouldn't be Andy Murray, though, if he didn't have the remedy for most things that can be thrown at him. While Karlovic has won two of his three meetings with Djokovic - the Croat is one of just three men to beat the Serb this year, back in Doha in January - the Scot has prevailed on all five of their previous matches. He generally brings the skyscrapers of the modern game crashing to earth, usually by dint of his supreme reactions on the return, and knack of causing them to pick up numerous balls from their shoelaces.

These skills are honed by asking his coach or hitting partner to try to ace him from just behind the service line for 10-15 minutes at a time. "I don't want to know where they are going to serve," he said.

While Murray and Karlovic haven't met across a net for fully three years, they do have some previous at Wimbledon. The Scot lost one set when they met here in 2012, but the main topic afterwards was the Croat being called for foot faulting 11 times, alleging some sort of conspiracy and subsequently asking for an apology from the All England Club.

"Well it wasn't a problem for me, more for him, because when he gets foot faults he loses the first serve," recalls Murray. "But it was strange, because he, and very few of the players, foot fault on a regular basis.

"Look it helps when you've beaten players before but it doesn't necessarily make the match any easier because he's an extremely tricky guy to play against, and he's beaten some very good grass-court players here," added Murray. "Obviously in Halle his results were good. He beat [Tomas] Berdych there, he's beaten [Jo-Wilfried] Tsonga here, so he's very dangerous."

Karlovic didn't earn a single break point against Tsonga yesterday, but such details matter less when you can serve the way he did in Halle. The 45 aces he managed there in his quarter final defeat of Tomas Berdych set the world record for a three-set men's match. Across the continent at Queen's Club, the achievement was the talk of the locker room.

"I was following the match on the live score app, I think during one of the rain delays," said Murray. "I was thinking that's got to be close to one of the records because he was already on like 30-something. Everyone was talking about it. They were pretty amazing numbers."

But enough about Karlovic's serving arm. Of more concern to most is the state of the Scot's own right shoulder, a problem he revealed yesterday that he had been carrying for "two to three" days. He may be the last Brit standing for the eighth successive year, but he spent part of Saturday lying prone on the turf, his bones cracking "like a machine gun" under manipulation by the physio. A rest day yesterday was timely indeed, and there certainly was no panic from Murray, who said it wasn't "a major concern".

Assuming he remains healthy, the Scot's progress through the draw appears clear enough. Yet while many of the men who would traditionally have been perceived a major threats - David Ferrer, Rafael Nadal, Tsonga - have been cleared from his path (indeed Murray can face no-one seeded higher than Viktor Troicki at 22 before the semi-finals) there are plenty of players of big threats left in the draw, literally. Hot, dry conditions are likely to help the big servers further, but Murray doesn't buy into the theory that Centre Court is playing any faster than usual.

"In the top half you've got Novak [Djokovic] and Stan [Wawrinka] and [Marin] Cilic is still in there, Kevin Anderson's a very good grass court player," said Murray. "Roger [Federer] is in the bottom half and around where I am there's a bunch of guys that are 6ft plus. I mean Karlovic is huge but Pospisil and Troicki are both 6ft 4in, pretty big guys, there's a lot of big guys with big serves still around. I don't think Centre Court is playing unbelievably fast compared to usual but when the temperatures are up in the mid-30s it makes it quick."

The man to consult about such matters is Karlovic's father Vlado, a meteorologist, but typically for a doctor, Karlovic can occasionally have you in stitches. The oldest man to reach the last 16 here since Niki Pilic in 1976, the 36-year-old is an irreverent figure on Twitter. One tweet saw him link a picture of Keith Richards and the young Andy Murray along with the sub-heading 'Rock Stars, then and now". "He is quite quiet but he obviously has a fun side in the way he celebrates matches and stuff," said Murray.

"It will not be easy," said Karlovic, who puts his longevity down to the fact that he was a late starter on the tour. "But if I continue serving like this, I will definitely have an opportunity. I'm just having fun. It is unbelievable that I am able to do this at my age."

In addition to being a serving specialist, the Croat also knows his way around a tie-break. He has played a whopping 563 tie-breaks in his career, winning just five more than he has lost. In the calendar year of 2015 alone, he has played 35 to the Scot's 15. Half of all their sets together have ended in tie-breaks but Murray has won four to Karlovic's three. There may be more to follow today, but an eighth successive Wimbledon quarter final would be just what the doctor ordered.