The tourists, replying to Australia's first-innings 570 for nine declared, had already lost captain Alastair Cook to a Mitchell Johnson thunderbolt the previous evening.
After an hour's play, they had lost two more big wickets and faltered to 75 for three.
Root and Michael Carberry began their innings with a combined deficit of 243 runs to offset, having dropped Michael Clarke and Brad Haddin early in their respective centuries.
If that was a harsh calculation, by the same token it was to the credit of both that they did not fall prey to any adverse psychology as they got themselves in again.
Sadly, for Root, the hard work did not pay off.
He played a pre-meditated slog-sweep at Nathan Lyon, to the first ball he faced against the off-spinner on a sunny morning.
After watching Haddin sweep England to shreds on day two, it must have been especially frustrating for the young Yorkshireman to mistime off the upper edge of his cross bat straight to deep square-leg.
Root and Carberry had impressively seen off Johnson and Ryan Harris' first spells, taking nine runs from the latter's final over after a necessarily watchful start.
Root's departure brought Pietersen into the fray, at a venue where he has made a century and a double-century on the last two tours here.
This time, he was still runless when Australia went to DRS - to no avail - over an appeal for caught-behind by Peter Siddle.
But the same bowler was to get him in his next over, Pietersen falling into a very obvious trap as he whipped a length ball to a juggling George Bailey - one of two fielders posted for the catch at midwicket.