There must be a few sports psychologists out there who would like to put Novak Djokovic through a CAT scanner. What happens in the world No 1's brain when he is under threat on the court? Whatever it is, you can bet that every other athlete in the world would love to know.
Yesterday's match at the O2 Arena reminded us how difficult it is to bring this man down.
In his seventh match against Djokovic this year, Andy Murray opened up with a set of near-perfect tennis, a set in which he dropped just three points on his own serve and hammered his forehand as confidently and rhythmically as a man driving a stake into the ground.
What did Djokovic do? He breathed deeply. He bided his time. And then he began turning up the pressure, little by little.
Murray might have beaten him in two of their most significant meetings this year -- in the final of the US Open and the semi-final of the Olympics -- but that doesn't mean he has established any kind of ascendancy in their 14-year rivalry.
After Djokovic's 4-6 6-3 7-5 victory yesterday, Murray is trailing by 10 wins to seven, and by four to three this season.
The good news is that, if you are going to lose to Djokovic, this is the time to do it. Murray came into this match with one win already under his belt, and the round-robin format means that he is still favourite to go through to the knockout phase.
The bad news is that, by ceding the top position in Group A to his chief rival, Murray now has every chance of meeting Roger Federer in the semi-finals. And that is the draw nobody wants.
Federer is aiming for a third straight ATP World Tour Finals title, and he saw off Janko Tipsarevic on Tuesday with the dismissive air of a man telling his butler to stand down for the afternoon.
Murray was commanding over the first 45 minutes yesterday. This was a match decided by small margins, so small that Murray attributed the result to the final two minutes.
But the momentum began to switch when Murray had a break point early in the second set. Djokovic came in behind his serve and hit a drive-volley bang onto the junction of two lines, right on the corner. He surely didn't mean to cut it quite so close, but it made for an unreturnable shot.
Was it jammy? Frankly, yes. But then Djokovic's X-factor is his ability to play with a loose elbow, as if just knocking the ball around in his back garden, when there are millions watching on TV. His ability to hit the lines at the big moments is uncanny.
"I try to follow my instincts," he said, "and play the shots that I feel are right at that moment." (© Daily Telegraph, London)
World Tour Finals,
Live, Sky Sports 1/BBC2, 12.0/6.0/1.45