Pietersen dug his team out of a hole by hitting 113 his 23rd test hundred but was one of three batsmen removed after tea by Australia, which reduced England to 294-7 in reply to 527-7 declared. (Agencies)
If the English lower order, starting with Matt Prior (6) and Stuart Broad (9), can get the team past the follow-on mark of 328, Australia's hopes of victory to keep the series alive would be seriously dented with only two days left.
It means the opening hour's play on Sunday could define the series, with England only requiring a draw to retain the Ashes urn. Weather could also be a factor, with some rain forecast on both days.
Ian Bell continued his fine form this summer by scoring 60 in a crucial 115-run stand for the fifth wicket with Pietersen, and Alastair Cook added 26 to his overnight tally to make 62, but the day was all about Pietersen.
The South African-born batsman was only playing here after passing a fitness test on a left calf injury sustained in England's victory at Lord's two weeks ago.
There was a train of thought that he should be rested and kept fresh for the final two tests of the series the selectors will be thankful they kept faith with their superstar batsman now.
Few batsmen have the ability to take a game away from an opposition like Pietersen can. That's why the ECB went to such great lengths to bring him back into the fold after the damaging and drawn-out phone-message row that marred last year's test series against South Africa.
He came to the crease with England stumbling to 64-3, following Jonathan Trott's dismissal for 5, and made a jittery start, wafting at a couple of wide balls and nearly getting bowled. Once he got settled, though, it was a masterclass from a batsman who just loves the big occasion.
Pietersen dug his team out of a hole by hitting 113 his 23rd test hundred but was one of three batsmen removed after tea by Australia, which reduced England to 294-7 in reply to 527-7 declared.