Isner on the board. The American holds both of his serves, the second a 134mph serve which Murray can't return to safety.

Timely. Murray smashes down a 126mph ace. In control. Back to the Isner serve...

Breath out Andy. Isner pounces on a short forehand, but lets Murray off the hook. Isner into the net.

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Mini-break. Isner second serve, long rally, Murray pumping the Isner backhand. Then Murray switches sides and the American clubs one long.

One more service game for Murray to negotiate...then it is tie-break time. The second seed does just that, Isner wincing as he slaps a weak double backhand into the tape for 40-0. Muzza smashes down another rapid delivery to the backhand...game over. Tie-break it is...

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Nope, not happening yet for Andy Murray. John Isner's serve is proving harder to break than a pair of frozen fish fingers. The American has landed 74% of first serves, perfectly illustrated by a 200kph ace down the middle for 40-15. That is followed by another too-hot-to-handle delivery, Murray sticking strings on it, but not enough to direct back over the net.

Mike Dickson
Daily Mail tennis correspondent on BBC Radio 5 live sports extra

"There's quite a bit of tension because Murray can't afford to make one false move on his serve against an opponent who has such a good serve."

You know we've been banging on about Isner's serve? Well Andy Murray isn't bad with the ball in hand either. Muzza slams his third ace of the afternoon - level with Isner - for a 30-0 lead, then sees a forehand from Isner just miss the whitewash where baseline meets sideline. The umpire hops down to check...out.

As Daily Mail tennis expert Mike Dickson pointed out on 5 live sports extra earlier, John Isner isn't just a serving machine. There's some finesse in those shovel-like hands too, you know. The American shows both sides of his game here, leaving Murray nowhere near another sniff of a break.

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Still on serve then on Lenglen. Murray will be thinking more positively after turning some heat on the Isner serve. Now, can he hold his own without any scares? He can. The Scot races into a 40-0 lead, has a little blip when he steers a backhand wide, then clinches it after a wayward Isner forehand.

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Isner has not lost a point on his first serve yet. This would be a decent time for Andy Murray to ruin that statistic...

And he's got a chance. Murray piles pressure on the Isner serve for the first time today...deuce. Isner attacks the Murray forehand, leaping forward to whack a forehand winner. Murray isn't going quietly though, making the American start again at deuce. 

This time he sees the job through, sealed with an...ace. Of course.

Image source, AP

Mike Dickson
Daily Mail tennis correspondent on BBC Radio 5 live sports extra

"This is a different match to the Karlovic one where very few points went beyond four strokes."

Danger averted. Another service game, another Isner break point. Murray slopes forward as he has Isner scrambling at the backhand side of the baseline at 30-30...oh my...he's planted his drop volley at the net over the sideline. Murray looks to the heavens. Focus Andy. That he does, cracking an ace down the T. A sumptuous double-handed backhand, crosscourt and kissing the whitewash, gives Murray advantage and Isner buckles finally buckles again.

Image source, AP

A quick breather as Isner has the trainer on. He has a blister on his middle finger apparently, hence why the American was fiddling with his digits in his seat earlier. No major problem. He's ready to go...

Andy Murray hasn't stuck racquet on ball too much when facing the Isner serve. But he does here, a rasping backhand putting him 15-0 ahead. From then on it is all one-way traffic though, Isner ticking over his side of the scoreboard nicely.

Image source, AP

Mike Dickson
Daily Mail tennis correspondent on BBC Radio 5 live sports extra

"Isner can trade groundstrokes quite happily so it's unfair to cast him as a one-trick pony."

Uh oh. Isner has a chance. The 15th seed claims the match's first break point, only for a wild return to fly over the baseline. Murray roars, Isner groans. The Briton isn't over the line yet, though, Isner cracking a sweeping forehand past his opponent that dusts the baseline. That forces a second deuce. Murray digs deep again though to wrap up a gutsy hold.