But team officials corrected earlier statements and said Root was instead weakened by the effects of a gastro bug.
Root came out to bat on the dismissal of Moeen Ali for 13 an hour into the final day's play.
Root was sent for checks after experiencing diarrhoea and vomiting overnight.
The skipper, who has reached his fifth half-century of the series but has yet to reach a century, is seen as England's last major hope of staving off a 4-0 series rout.
His plight evoked memories of Englishman Eddie Paynter, who got out of his hospital bed to play an heroic four-hour innings of 83 in Brisbane to help England overhaul Australia's first innings in the 1933 Bodyline series.
Spinner Nathan Lyon claimed Moeen's wicket for the seventh time in the series when he trapped him leg before, which Moeen did not review.
Lyon had taken three wickets for 50 off 32 overs at lunch.
Cameron Bancroft, fielding close in at bat-pad, took a full blow from Root on the helmet grille and hardly flinched before seeking replacement headgear.
The Australians took a grip on the final Test with a massive 303-run innings lead and then reduced the battle-weary tourists to 93 for four at the close on the fourth day.
All five Tests in the series have gone to the fifth day.
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