Murray's level of maturity continues to rise in Melbourne

The 22-year-old fifth seed perfectly executed his game plan to nullify the imposing threat of the 6ft 9ins American's serve and claim a 7-6 (7/4) 6-3 6-2 win.

It was an unyielding display of all-round tennis from Britain's No 1 as he made just eight unforced errors in a shade over two hours on the Rod Laver Arena court. Murray will next face defending champion Rafael Nadal of Spain, but in the meanwhile he was content to focus on his performance against Isner which had loomed as a tight affair when the Scot had to fend off a set point in the first set before taking it in a tie-break.

It proved a key moment as Murray's superior fitness told thereafter and he ran away with the match.

Afterwards the Scot said his ability to outwit his opponent at crucial times had been pivotal to his success, especially when he was tested early on.

"A lot of that comes with experience, knowing what to do on the important points, the best way to go about playing certain opponents," said Murray who joined the ATP Tour in 2005.

"When I was down (against Isner) I hit a lot of winners, served smart. I was just thinking the whole time.

"I guess it's a maturity thing. You learn to deal with the tight situations better and your focus holds for longer.

"Sometimes in the past, when I was sort of 18 or 19, I might have been getting worked up or not thinking about what I should be doing on the next point and worrying about what happened in the past.

"Now, it's just focus on each point (and) so you make the right decision more often."

Murray has been feted more recently for his build and physical condition which he has sought to improve to last five sets.

But he proved he can play a patient game as he waited 10 service games before getting his first break of serve.

There were some signs of frustration during that time but with Isner not pressing Murray's serve – he lost just two points in his opening five service games – the Scot was able to focus on the American's delivery.

Slowly but surely he got to grips with the 125mph thunderbolts coming his way and broke three of Isner's last five service games to dispirit the American to such an extent he did not bother chasing the drop shot that finally sealed his fate.

"I'm playing well. There's no question about that," confident Murray said.

"I thought it was a really good performance. I focused well, concentrated hard, and served well. Everything was good."

The win continued Murray's trouble-free passage to a first quarter-final at Melbourne Park – the Scot has not dropped a set along the way.

Murray is, however, wary of such statistics after he did not surrender a set before losing to another Spaniard, Fernando Verdasco, in the fourth round last year.

"I've played it through to the fourth round a few times here without dropping a set," said Murray, right.

"I don't know if that means much – I just played well today. I've concentrated well in all of my matches.

"That's really it and I hope it continues."

Nadal had a similarly imposing task to that of Murray, against Croatia's Ivo Karlovic, who stands one inch taller than Isner, and while the Spaniard dropped a set he did enough to win

6-4 4-6 6-4 6-4.