By Clive Everton BBC Sport at The Crucible

The agony shows on Steve Davis' face

There is no sport quite like snooker for demanding the capacity to withstand frustration.

This comes in many forms, the most basic of which is a player's inability to produce the form he knows he is capable of.

In press conferences, player after player will say he is playing well in practice while being able to bring only a fraction of that form into the arena.

Snooker is a still-ball game, played in a restricted 12ft by 6ft area, with no variables of climate. Top players practice on tables very similar to championship tables.

It offers a mirage of perfectibility - as billiards did in the 1930's when the top players became so good that they killed the game as public entertainment.

In snooker, a bad shot or a fluke can lead to a player sitting fuming and powerless in his seat for the next 10 minutes

It is a shock to their systems when snooker players make more simple mistakes in a match session than they would make in a month in practice.

A realist like Steve Davis says: "In my experience, people don't generally play as well in matches as they do in practice and it's dangerous to consider practice form as a yardstick."

The absence of pressure or fear in practice sessions accounts for this difference.

But sometimes this disparity is just too much to bear.

At The Crucible last year, Jimmy White impulsively conceded a frame with a double hit when a glaring error, after many others, was more than he could stand - his only breach of perfect snooker etiquette in 22 years as a professional.

This year, Chris Small conceded a frame against Matthew Stevens when only 24 behind with five reds left.

He had practiced so hard and the match meant so much to him that when he missed a ball he could probably have potted with his eyes shut, he lashed out at the balls and stalked out of the arena oblivious to the fact that he could still win that frame.

"I shouldn't have done that," he acknowledged. "If someone had done that against me I would have been quite annoyed. It's just frustration knowing that I was blowing my chance. I really wanted to get out of there."

Matthew Stevens reacts after missing a pot

In sports like tennis, a bad shot or a lucky one from your opponent costs you one point and there is a new start at the next point.

In snooker, a bad shot or a fluke can lead to a player sitting fuming and powerless in his seat for the next ten minutes.

The trouble is that there is no scope to discharge this frustration with any form of exertion as there is in most other sports and it does not help, as Davis puts it, that the television cameras are up your nose.

Davis himself had to endure the frustration this week of Stephen Lee's safety game being so tight that he did not pot a ball for 70 minutes.

Quinten Hann's basic problem is his low tolerance of frustration.

When he fell behind against Lee last year, he took to smashing the reds open indiscriminately from his break-off shot thus hastening his demise, instead of somehow summoning the will to battle it out to the end.

Championship snooker is the ultimate mind game in which the constant challenge, in Davis' classic phrase, is "to play as if it means nothing when it means everything".