The next time you hear a sports commentator say: “It’s all about who wants it most”, I suggest you spit on the floor. Quite apart from it being bad grammar, it is the exact opposite of the truth. Yesterday, there was no doubt that Roger Federer wanted it more than his opponent. The wanting came close to destroying him.

It was the wanting that had him at times in total disarray. The cool certainties, the serene accuracy, the profound understanding of the tactical ebb and flow of a game: all these things went out of the window in the face of Federer’s unspeakable and colossal desire.

It was wanting it more that made him vulnerable. The certainty of what he wanted to achieve almost allowed everything he wanted to be plucked from his grasp by a strapping young giant who might have been hand-picked as a perfect Federer opponent. Juan Martín del Potro, all 6ft 6in of him, came close to blasting Federer to bits in the semi-finals of the French Open yesterday.

With Federer not only trailing, but playing agonisingly badly, it seemed not only the end of his campaign in Paris, but of his career as the greatest champion we have ever seen. But somehow, he turned the juggernaut of destruction around on the brink of the precipice and won the match 3-6, 7-6, 2-6, 6-1, 6-4.

The problem for Federer was that both players wanted different things. Admittedly, they could achieve what they wanted only by means of victory, but all Del Potro wanted was a place in the final, and a chance to win his first grand-slam title. Federer wanted rather more. He wanted, and still wants, to be as great as Pete Sampras and Andre Agassi combined. He wanted, and still wants, to win his fourteenth grand-slam title, the same number as Sampras. He wanted, and still wants, to win in Paris and so complete a career grand slam: a title at each of the four venues of the modern era.

So close, so close he could almost touch it, he could almost taste it. And that was the problem. With all the big guns knocked out by players other than himself, Federer knew it was all there for the taking. As a result, he couldn’t help getting ahead of himself. That’s what wanting it too much does.

It stops you playing points; it starts you thinking about victory. You find yourself playing in the wrong tense: sport can only be played in the present, in the Zen-like moment. When you want something too much, you start to play in the future tense.

And that is when troubles overwhelm you.

I have never seen Federer play as badly as he did in the third set. He had just forced himself into the match against a man who refused to allow his serve to be broken even once: prevailing in a tie-break in which he at last began to play with authentic Federer-like conviction and brilliance.

But he threw away his next service game like a wet-behind-the-ears novice. He made errors in every area. He hit the ball with the frame of the racket. His mind became a complete tactical blank. He was playing on nerves and emotion: each point was a disaster waiting to happen.

In short, it was like watching Tim Henman: a too-great partisanship becoming merged with a too-great anxiety, an anxiety that was fully justified. It really did feel like the end of it all: that the great wizard of our time, the Harry Potter of tennis with his racket made by Ollivanders wand company, was about to lose everything. All because he wanted it too much.

Federer has been having a dreadful tournament. The only saving grace is that he keeps winning. But he had consistently been unable to assert his authority — that authority that was once the keystone of his game — over any of his opponents with any conviction. It is Federer who has been dominated: by the weight of history, by the extraordinary things that he may just be on the brink of achieving.

Perhaps it all turned on a single point: a drop-shot of maximum disguise and surprise, one that fell to the red dirt like a shot bird, leaving Del Potro stranded. That gave Federer a break point in the fourth set — and the match had turned. Not that Del Potro gave up: he is 20, and full of muscle and fight. He blasted away right till the end, and never looked anything other than a player who was in the semi-finals by right.

More than you could say for his opponent. But Federer survived, and there was enough stuff from the real Rodge to make you look forward to tomorrow, when he will play a selection of opponents in the final. He will play Robin Söderling, a manic Swede who has been peppering the lines like a man in a dream.

The other opponents are probably even stiffer: Sampras, Agassi, history and, toughest of all, the unhidden desires of Roger Federer.

Federer knows that Söderling is eminently beatable by anyone who can play remotely like Federer. Trouble is, he knew the same thing about Del Potro. An afternoon of dreadful anxiety awaits. And if it’s all about who wants it less, Söderling is home and hosed.