THIS was a case of job done and on to the next one for Andy Murray.

His third-round match last night against Andrey Kuznetsov, the world No 97, wasn't as emphatically straightforward as it had looked when the Scot powered into a two-set lead, showing no ill-effects from the painful cramping episode which almost sent him crashing out in the first round on Monday.

For although Kuznetsov might not have possessed the kind of power game to knock the Scot down, he did have the ability to stay in a match which was rapidly getting away from him. His longevity was evident in the previous round when Fernando Verdasco was felled in a gruelling five sets and that came to the fore here.

Murray, out of nowhere, began to wobble, yet held his composure to set up a date with either Jo-Wilfried Tsonga or Pablo Carreno Busla in the last 16 after triumphing 6-1, 7-5, 4-6, 6-2, in two hours and 35 minutes.

The stats certainly showed Murray's willingness to be the aggressor, the 27-year-old winning an impressive 29 from 33 points at the net while hitting 47 winners to the Russian's 33.

A tally of 70% of first points won also indicates a strong service game in quite humid conditions.

This US Open, though, is about to get very serious for the Scot, who would have duly noted that Novak Djokovic, the world No 1 who he could meet in the last eight, is back to his imperious best after destroying Sam Querrey in straight sets.

Murray, who has not beaten a top-10 player since defeating Djokovic in the Wimbledon final last year, knows his game will have to be upped if his magical run here of two years ago is to be repeated and his barren trophy spell ended.

Yet Murray looked great early on against a player who came into this match having superbly beaten Verdasco, as his fine run of form which saw him similarly upset David Ferrer at Wimbledon continued here in New York.

He was moving superbly, his second serve - not normally a strong aspect of his armoury - was working nicely and his aggressive game plan was leaving his opponent crestfallen.

Kuznetsov looked nervous and took a while to get into his stride. By then, though, Murray had already broken for 3-0.

It wasn't as if the Russian didn't have any opportunities, rather that he was unable to finish Murray off in a number of close games which never really convinced there was a chance of a shock victory.

A far too simplistic break for 5-1 was administered by the 2012 US Open champion and, following a superb rally in the next game which showed Murray at his rampaging, attacking, powerful self, the first set was duly pocketed.

An early break at the start of the second ensured the worst possible start for the Russian who, up until that point, was unable to take any of the three break points which came his way.

Yet, having fallen 3-0 down, suddenly Kuznetsov looked reborn. The key game was at 3-4, a double fault from Murray gifting his opponent the chance he was desperate for. For the first time, it was the Scot who was under pressure. Thankfully, the wobble was just that.

A double break point opportunity flashed up and Murray needed no invitation to step into the ball and crash a forehand winner with his unerring accuracy to reassert his authority with some pumped up brilliance.

An ace handed Murray a set point and another superb serve into Kuznetsov's body ensured the world No 9 sat down for a drink of water, looked over to his player box and clenched his fist in excitement.

With what lies ahead, it was vital to get the job done. And quickly, and Murray was now in no mood to waste time or energy. He was in total control. Or so we thought. A scratchy game which saw the Scot chunter and growl to himself ended with Kuznetsov breaking to stretch ahead 3-1.

He suddenly looked out of sync with himself, a quite awful, head-scratching set which ended with a double fault had, out of nowhere, given the Russian something to build on as Murray lost the third set 6-4.

However, Murray was quickly ahead in the fourth set, breaking for 2-1 when Kuznetsov drilled a forehand over the baseline.

The Russian was under real pressure and, although he saved two break points in the fifth game, one with a brilliant drop volley, he then threw in successive double faults to hand Murray what was to prove a match-winning advantage.

Still, it was not entirely straightforward as Murray immediately surrendered one of the breaks, but he got it back quickly and took his first match point with a reaction volley.

Now the real business starts.