A statement posted by England Cricket on Twitter last night read: “Joe Root is currently in hospital with severe dehydration. There will be a further update as to whether he will be fit to resume this morning.”
That update later read: “Joe is now on his way to the ground. He is unlikely to start this morning when play resumes.”
Root's bravery was in vain as England slid to an innings-and-123-run defeat in the final Test to complete their miserable Ashes campaign as 4-0 losers.
Root spent much of the morning in hospital suffering the effects of severe dehydration and was unable to rejoin his team's improbable rearguard at the SCG as they resumed on 93 for four - still 210 runs short of making Australia bat again.
The England captain did continue his innings an hour later, after his replacement Moeen Ali was dismissed, but could not do so again following the lunch break as the tourists subsided in his absence to Pat Cummins (four for 39) and finished 180 all out.
Sydney experienced its hottest weather in 79 years on day four (Sunday), with temperatures reaching 47.3C. Root was in the field for much of the day before batting for almost three hours before the close.
Meanwhile, Root will take part in next month's Twenty20 Tri-series against Australia and New Zealand, but Moeen Ali and Bairstow will both be rested.
England have named a 16-man squad for the mini-tournament, which begins in Hobart on February 7.
As expected, it also contains Ben Stokes - although his participation remains unlikely while his wait continues to hear whether he will be charged with causing actual bodily harm in a fracas outside a Bristol nightclub on September 25.
Fast bowler Mark Wood and batsmen Sam Billings and James Vince are back in the reckoning in the sprint format, having missed England's last Twenty20 international against West Indies at Chester-le-Street four months ago.
It was initially anticipated Root would be due to have a short rest after leading England in the Ashes - set to end in Sydney on Monday - with the tourists battling to avoid a 4-0 series defeat.
He has been included, however, unlike all-rounder Mooen and wicketkeeper-batsman Bairstow, two other regulars across the formats, who like Root are part of the one-day international squad which will take on Australia in five matches this month.
Batsman Dawid Malan, another Ashes mainstay set to play in the ODIS as well, is also in the Twenty20 squad.
Chairman of selectors James Whitaker said: "Our aim is to become the best Twenty20 side in the world, and a strong performance in this series will give us the chance to progress up the team rankings.
"We felt it was important for Dawid Malan and Joe Root, who have had an intense period of Test cricket over the past few months, to play in the series and continue to develop their Twenty20 experience and skills."
There is once again speculation, meanwhile, that Stokes may be about to hear very soon whether the Crown Prosecution Service has advised police to charge or clear him.
In a brief statement accompanying confirmation of the Twenty20 squad, the England and Wales Cricket Board reiterated its current policy regarding the key all-rounder's selection.
It read: "Ben Stokes is included, though his involvement remains subject to any relevant legal or disciplinary developments in relation to the incident in Bristol in September.
"Should the ECB board receive formal confirmation that Stokes has either been charged or that he will face no charges, they would convene within 48 hours to make a decision on his availability for the team at that stage."
England T20 Tri-Series squad: EJG Morgan (Captain), SW Billings, JC Buttler (wkt), TK Curran, LA Dawson, AD Hales, CJ Jordan, DJ Malan, LE Plunkett, AU Rashid, JE Root, JJ Roy, BA Stokes, JM Vince, DJ Willey, MA Wood.