Ashes 2013: Kevin Pietersen is England's rock at Old Trafford

Kevin Pietersen has scored hundreds that were more aesthetically pleasing, but his 113 on day three of the third Test at Old Trafford was one of his finest given the circumstances.

England were on the ropes before lunch so Pietersen had to score a hundred and he did. He also had to work hard for it, and when Pietersen has to work hard, somehow it makes for more worthwhile viewing. And the fact his side really needed that knock will give him great satisfaction.

He has not had much time at the crease recently and he was passed fit only on the morning of the game, and when he came out to bat you could tell.

When he comes to the crease he can be incredibly skittish, play some astonishingly loose shots and get himself into some very strange places.

He looks terribly vulnerable - it's almost as if he does it on purpose, to make the bowlers think, 'hey, I'm going to get this bloke out'.

There is usually a mad run to get off the mark and suddenly he slips into a groove - it is as if a button is pressed and everything moves as it is supposed to.

And as soon as he is in the groove he is incredibly hard to bowl at. My generation had Viv Richards, who pumped the ball through the leg side from pretty straight, which was his great strength.

But Pietersen hits it straighter, even when the ball is short of a length, because he can stand tall, get on top of the bounce and hit it away from off stump through mid-wicket and mid-on.

As a bowler that is a nightmare, because if you bowl outside off stump he will hit you through the covers. The margin for error is miniscule. Plus, the captain and the bowler are trying to cover the leg side, so they end up with a very split field with gaps all over the place.

Pietersen is box office to watch. The way he saw off Nathan Lyon before lunch, by smashing him for a couple of sixes, was just terrific. But he had to work very hard for that hundred, on a pitch that is doing more than he allowed it to look.

Peter Siddle bowled really well in the channel just outside off stump, moving the ball away a touch, and Shane Watson beat Pietersen three times in a row, so it is still doing a little bit, especially when the Australia seamers land it on the seam.

Australia captain Michael Clarke did not do much wrong, although he might have bowled leg-spinner Steve Smith more - he only really brought him on as a token effort just before taking the second new ball.

English players do not play leg-spin very well and I would have liked to have seen Smith bowl at Pietersen. He would have wanted to get after him and, if he had gone for 25 runs in three overs, it wouldn't have really mattered.

Ian Bell is in beautiful form. That late cut of his, like a dab, is an old-fashioned shot that you do not see too many people playing nowadays. You can only play it if you are in very good form - you have to see the line and length early and control it, otherwise it is a get-out shot.

That afternoon session might turn out to be the one that clinches the Ashes for England. Australia had the door open at lunch but Pietersen and Bell closed it - although not completely.

Even if England avoid the follow-on, they are still likely to be plenty behind on first innings, so England fans mustn't think, 'phew, it's all over'.

There are two days left, Australia are chasing and if they do get the chance to bat again they must score quickly. That being the case, they might get bundled out, so lots of things are still in the melting pot.

England have taken another step but they have got a lot more batting to do. Ideally, they need to bat well into the fourth afternoon. That would leave Australia with nowhere to go in this series.

Jonathan Agnew was speaking to BBC Sport's Ben Dirs.