When someone sees one day the inside of Andy Murray's mind they will doubtless find the inner workings of a tortured genius but, thankfully, the genius tends to win the battle.

For the second time in his career, Murray is through to the semi-finals of the French Open, thanks to another example of his outstanding mental strength, his demons given the heave-ho just in time.

From a position of utter dominance at two sets to love up on Gael Monfils, Murray dropped his guard and found himself embroiled in a decider against the mercurial Frenchman with light running out fast. As he has done so often, though, Murray showed he is mentally stronger than most and recovered his poise to clinch a 6-4, 6-1, 4-6, 1-6, 6-0 victory, just before darkness fell at Roland Garros.

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It was as topsy-turvy a match as the score suggests but when it really mattered, Murray's mind was the stronger as he equalled his best effort at Roland Garros: a semi-final showing in 2011.

Tomorrow, as three years ago, it will be Rafael Nadal standing in his way and Murray knows he cannot afford the mental highs and lows against the eight-time champion if he is to stand a chance of reaching the final for the first time.

If ever a match summed up Murray, it was this one. For two sets, even though the wind was tricky, his game plan worked perfectly, he was in control of all his emotions and Monfils never had a chance.

Murray looked like he was intent on running the legs off Monfils, who admitted before the match he was tired and who was forced to run twice as much, almost in every point. The Scot's backhand was as accurate as he would have hoped and as he targeted the Monfils backhand, he was able to exploit the space on the forehand side whenever a ball fell short.

But, midway through the third set, Murray began to become irritated, chastising himself after every lost point and gesticulating wildly towards his support box.

Encouraged by what he was seeing, Monfils picked up his game and the Frenchman snatched the third set to stay alive.

Suddenly the crowd on Court Philippe Chatrier woke up and with Murray all over the place, a far more aggressive Monfils levelled the match and, at 9.20pm, Murray wanted the match to stop.

Somehow, the Scot dug deep, threw out his demons and, from 15-30 in the opening game of the final set, played three superb points to hold serve and move ahead again.

Having forced two break points at 15-40, Murray chased down a Monfils drop shot and then, when the Frenchman tried to go crosscourt, the seventh seed was there to put away the volley, prompting another fist pump.

From that point, the match was effectively over. Murray broke twice more to clinch a victory that Monfils could barely explain.

"I wanted to finish today because I knew that Andy was tired and I was better than him," he said. "I didn't win the first game when I was in a position to win it, and then I rushed it. I tried my forehand and my shots were out, and then it went very fast. I thought, 'okay, I need to remain aggressive.' It slipped."

Next up for Murray is Nadal, the man who denied him here three years ago and who recovered yesterday to beat his fellow Spaniard David Ferrer 4-6, 6-4, 6-0, 6-1 to move a step closer to a record-extending ninth French Open crown.

Perhaps there is nothing more dangerous than taking a set off the world No.1 on his favourite surface, on his favourite court in the world.

After an error-prone first set, Nadal broke Ferrer early in the second and held the advantage throughout the set, not without trouble, to level the match.

From 5-4 in the second set, Nadal won 10 straight games to win the third set and go up 3-0 in the fourth, settling into his rhythm at last and clinching a victory that, in the end, was fairly comfortable.

"Today I was not good enough for this match," Ferrer said. "I lost my focus. I was too slow and I think I didn't play the game of a top-10 player. This is why I'm sad. It's my attitude, my behaviour on the court.

A relieved Nadal said he was "playing amazing in practice" but then when he stepped on court against Ferrer, he "couldn't hit his backhand".

But he still won. And though Murray pushed him hard in Rome earlier this month, he remains the favourite to take the title again.