Kevin Pietersen scored a brilliant century here against New Zealand and then told England to keep faith with struggling teammates Paul Collingwood and Ian Bell.

But while Pietersen's 115 should be enough to see England take control of the third and final Test, his defence of two players who failed to score a run between them may not be enough to keep the selectors at bay.

Collingwood and Bell have the second innings in Nottingham in which to mount cases for continued inclusion in a side that has not changed for five matches.

Should they fail again, a set of six limited overs internationals against the Kiwis will offer a chance for the pair to hit themselves back into form. But if the team for next month's opening Test against South Africa at Lord's had to be picked this morning then at least one change would be difficult to avoid.

Not according to the loyal Pietersen, mind you. "It's obviously a struggle," said England's No4, who stood at the nonstriker's end as Bell and Collingwood came and went in seven deliveries.

"But you have a look at the top scorers in county cricket at the moment and none of those guys is better than Ian Bell or Paul Collingwood.

"You are going to have good days and bad days. Unfortunately they are missing out at the moment but Bell scored a hundred three Tests ago. And Collingwood, I know he's missed out, but he's an important member of our team. A great guy. It's only a matter of time.

"It is hard when I see people criticising these guys. Positives are what people should be talking about; like the way we beat New Zealand at Old Trafford last week. It's the positives I like talking about, not the negatives."

The four leading, England-qualified run-getters in the County Championship so far this season are Matt Prior, Marcus Trescothick, Mark Butcher and Ravi Bopara. Trescothick insists he has retired from the international scene and Butcher may be regarded as coming from the wrong generation.

But Prior, the only player apart from Pietersen to have made a century at Trent Bridge this summer, and Bopara, who reminded everyone of his prodigious talent two days ago by hitting a limited overs double century against Leicestershire, are giving the selectors a good shove, never mind a nudge.

Still, as Pietersen implored, let us be positive. The way England's best batsman and their tough little wicketkeeper Tim Ambrose transformed a perilous 86 for five into something far more comfortable with a stand of 161 was nothing short of terrific.

Ambrose had been going through a lean trot himself, scoring only 50 runs in five innings since taking a series-changing century off New Zealand in Wellington three months ago. And, as was the case in March, he walked to the middle yesterday with England facing a crisis.

"It was a scenario where we were staring down the barrel. Losing three wickets after lunch for two runs was not something we planned to do but those are the kind of times when I like to stand up for England. I like to score runs when we are in trouble."

Those comments came from Pietersen. But, on the evidence of what we've seen so far during Ambrose's brief international career, they might easily have been voiced by the keeper.

New Zealand's bowlers did not feed his favourite cut shot as they had done in Wellington but he was perfectly happy to wait for enough width to find ways through the off side and, when the Kiwis over-pitched, he drove handsomely on his way to 67.

"Timmy is a cool guy to bat with because he is pretty chilled, like me," said Pietersen of his partner in a stand which could clinch the series. Ambrose faced 148 deliveries, the last of which he edged behind to Iain O'Brien.

Pietersen also perished to the second new ball, in similar fashion, but having taken more than four hours to score his 12th Test century he deserved his standing ovation every bit as much for the patience displayed as for the power.

For New Zealand, it was re-opening an old wound. Ambrose thwarted them in Wellington and, in the final match of that series, Pietersen cut them deeply by hitting 129 at Napier.

Pietersen added: "The hundred in Napier, the Ashes century [158 at The Oval in 2005]; there have been quite a few I've scored when England have been in the dumps. It's something I pride myself on. I like to stand up to be counted on days when it's tough. It's something I thrive on."

Having reached 273 for seven, England returned to Trent Bridge this morning believing they were in a strong position. But a back nigglewhich prevented Ryan Sidebottom from batting at No9 will be a serious cause for concern if it inhibits his swing bowling today.