Root was taken to hospital on Monday morning and missed the start of play, not returning to the crease when play began at 11.30pm GMT.
Root was due to bat alongside Jonny Bairstow, having reached 42 not out in England’s second innings on Sunday evening.
However, after Root visited hospital on Monday, England Tweeted: “Jonny Bairstow and Moeen Ali start the day for us. Joe Root has arrived at the ground.”
He eventually came out to bat midway through the morning session after Nathan Lyon dismissed Moeen Ali lbw for 13, reaching his 37th Test fifty in the process.
Sunday’s temperatures in western Sydney were the highest recorded in almost 80 years.
In the suburb of Penrith, the mercury reached 47.3 centigrade, a figure not seen in the city since 1939.
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At close of play on day four, the tourists were 93-4, still 210 runs behind Australia.
Before batting on Sunday, Root had spent much of the day fielding, while Australia amassed 649-7 declared in their first innings.