SOMEWHERE in Manhattan, with his feet up Novak Djokovic must have allowed himself a big smile.

He would have already been in good spirits, content at having had the rub of the green on a few occasions already in the US Open, withdrawals through injuries allowing his sore left wrist to recover to something close to 100 percent.

But watching Andy Murray and Kei Nishikori go toe to toe over almost four hours with the stress of the situation adding to the exhaustion, that must have been the icing on the cake.

Not only did his big rival, Murray, go out, but in beating him, Nishikori expanded so much energy, mental and physical, that he will surely find it hard to back things up in his semi-final.

But as well as Nishikori played from the middle of the first set onwards, Murray will be kicking himself after letting slip a match he surely should have won.

Not only did he lead by a set and a break, but from two sets to one up, 1-1 in the fourth and break point up, he allowed his concentration to go to the extent that he lost seven straight games.

Still he had chances only to drop serve at 5-5 in the fifth, perhaps the efforts of the summer finally catching up.

A Wimbledon title and a second Olympic gold were outstanding achievements but a second US Open title will have to wait.

Murray, crunching huge groundstrokes and ripping into the Nishikori serve, led by a set and a break and the Japanese looked outclassed, as he had been at the Olympics when Murray won in two straight sets.

Sixth seed Nishikori, the runner-up at Flushing Meadows two years ago, had won just once in their eight meetings, an indoor clash in the ATP World Tour Finals in 2014.

And the lack of confidence in the match-up showed early on as he missed a series of easy forehands while his serve was being pummelled by Murray on the return.

Murray raced through the first set and led 3-2 with a break in the second only to be broken back and then came the first turning point – for there were a few – when the roof was closed because of rain.

Murray had been in total control but the change allowed Nishikori to get his timing and as he became more aggressive, he enjoyed more success.

Another break gave him the second set but Murray regained his composure well enough to take the third set.

At 1-1 in the fourth set, with Murray holding break point on the Nishikori serve at 30-40, the second turning point came. This time, a loud noise, from quite where nobody could be sure, caused the umpire, Marija Cicak, to call for the point to be replayed.

Murray was in charge of the point and had looked set to clinch the break and the choice of the umpire did not go down well with the world No.2.

After losing the replayed point, Murray approached the umpire to tell her that the same thing had happened earlier in the match but that she had said the point would not be interrupted.

Whatever the rights and wrongs of the discussion, Murray let it get to him and from that moment on, he was in trouble mentally, unable to regain his focus.

After a long summer – with just two losses since May – the physical effects had been nagging away at Murray throughout the fortnight.

Suddenly, points and games were disappearing fast and from 1-1 in the fourth, Murray lost seven straight games to find himself two sets all and down a break at 2-0 in the fifth.

Nishikori’s confidence was up, not surprisingly, and the Japanese was thrashing the ball, winners on both sides, lifting his aggression and coming to the net whenever he could, while hitting the odd drop shot to devastating effect.

Murray’s concentration, though, was shot. Ranting toward the umpire and his player box, where Ivan Lendl sat, stony-faced as ever, the Scot took a good 20 minutes to regain any semblance of focus.

A butterfly which hovered around the net for several games didn’t help either and at 2-0 in the fifth he was in serious trouble.

Just like that, he got his focus back and after a big hold in the third game, he broke back to level at 2-2, the come-ons increasing in volume and regularity.

At that stage, he would have been a big favourite to go on to win but from 40-15, he was broken as Nishikori continued to go for broke, coming forward brilliantly and taking time away from Murray.

But at 4-3, 40-0, Nishikori twitched, dropping the first two points and then, at 40-30, having worked the point superbly, with the court wide open, he dumped a sitter of a volley into the net.

That was the lifeline that Murray needed but again, he fluffed his lines, dropping serve at 5-5 when Nishikori hit a volley winner at full stretch and then seeing the Japanese hold on to reach the last four.