If there is one facial movement Ivan Lendl finds excruciating, it is that upward of curl of the mouth that spreads into what might loosely be described as a smile. When it is accompanied by an ever-so-slight moistening of the eyes, Old Stone Face is hurled into a maelstrom of embarrassment.

All of that happened in New York on Monday night. He couldn't help himself. He stood in Andy Murray's box, positioned like a royal enclosure, raised slightly above the Arthur Ashe Court on which Murray had just created tennis history – and lost it. "That was definitely a smile," Murray said.

The man who had guided Murray to his first grand slam title in just nine months was rightly proud, but the biggest struggle for him was to contain his emotions. This fighting of sentiment is part of his core philosophy of life, one he has imparted to the Scot, whose psyche has dangled on thin string for so long.

But Lendl is never tempted to live a late career as a coach through Murray. He won eight grand slam titles himself. He's done it all already.

"Well, that's why I came on board," he said later, "to help Andy win. He has won two big ones [an Olympic gold medal, where he beat Roger Federer in straight sets, then this US Open, where he ground down Novak Djokovic], it's a fantastic year, I'm very happy for Andy. It's a great achievement for him and let's hope he can continue, and rack up many more.

"I'm 52 years old. I've been away from tennis for 14 years. I don't look at myself as a tennis player, it's somebody that's …it's so long ago, I don't even think about it. I said the goal was for Andy to win majors and he had two fantastic tournaments this year and I'm very happy for him."

Yet some things will remain locked away. Asked what was the most important improvement he had brought to Murray's game since they started working together before the Australian Open in January, he bridled, a fall-back position.

"I'm not going to discuss that. He still has a career to go, he still has to play matches against these guys and, if I tell you what we worked on, what we planned to work on, if I dissect any of the matches, I'm giving away stuff. And, as you saw, the margins are so small that giving away something which would help somebody with two points in the match would be suicidal."

That's why Murray hired Lendl. He brings hard-headedness to the job, a work ethic and calculating approach that Murray can identify with. While others fretted towards the end of an extraordinarily physical struggle against Djokovic, Lendl stayed calm. Murray said later that detachment from the roaring atmosphere transmitted itself to him on the court.

Lendl said: "He started looking, in my mind, better than Novak half way through the fourth set. I thought then there was a chance Andy could come back in the fourth as well. He was a bit unlucky on some big points; Novak played very well on some big points and the fifth set was in front of us."

He agreed, after a quibble about the nature and purpose of the question, that Murray could win more titles. "I thought that when we started, yeah."

Asked if he was surprised how quickly they had gelled as a team, with a slam title to show for it after only nine months, Lendl gave it the full Czech stare: "I think your memory is very short. Or you have selective memory. Or it's just short. Both Andy and I were saying, 'Give us six to nine months'. Do the math. You can help somebody, obviously, in a very short period of time, however it takes longer than that to help more than that, for things to set in. You cannot do that in one week, you cannot do that in one month and, hopefully, we're not anywhere near where Andy can get."

And the next stage of the grand plan? "I'm not going to talk at all about the details. Again: listen to my words." But he was happy to expand on what had gone before. "We all know that it's a war out there. At some point it's very unlikely you're going to win a match in a blow-out. Especially against a guy like Novak.

"At some stage, it's going to come to who wants it more or how badly do you want it. I don't want to say that Novak didn't want it – you British guys, I have to be very careful with. So, how bad do you want it? What price are you willing to pay? And who can execute under extreme pressure? It's just a war out there. You've see both of those guys: they are totally spent."

Lendl said the simple lesson Murray learned at Flushing Meadows was what it is like to win a slam. "I wasn't there for the Olympic final. We talked over the phone almost every day and so on but Andy has been maturing very nicely as a player, as a competitor and as a person. And as you mature, you become more comfortable in these situations.

"The more of them you are in, the more comfortable you feel. To me, one of the most important matches of the year, maybe the most important, was his loss to Novak at the Australian Open. Because that was a war, just like tonight. And that has given him the belief that he can hang with these guys. It has also showed him what it takes him to win so it doesn't catch him by surprise."

Was there anything he discovered about Andy that surprised him? "No," he said, "except that his sense of humour, maybe, is as sick as mine." And, with that, an unequivocal grin played across his weathered, knowing face.