Joe Root on standby to open for England in first ODI against South Africa

Roy was able to bat in the nets on the eve of the first ODI, unlike 24 hours earlier, but took only limited part in fitness drills on the Mangaung Oval outfield.

The opener has been nursing a bad back, having suffered an unaccustomed spasm, and England have already worked out contingencies in case he does not recover in time.

Captain Eoin Morgan ran through them at his preview press conference, and appeared to favour the possibility of Root moving up from number three to open for the first time in this format - with England's utility batsman James Taylor then getting a game after all, at number three.

It is harsh on Taylor that there is no obvious position for him in England's current first-choice XI, even though his century in Saturday's warm-up win over South Africa A in Kimberley followed a sequence of more than 300 runs in his last seven ODI innings - at an average of 60.6.

Roy's readiness, it seems, will determine whether Taylor scrambles into a team welcoming back both Root and Ben Stokes - the men missing to accommodate the diminutive batsmen in each of England's last two series.

On the evidence of half-an-hour of batting and short-lived exercise beforehand, during which Roy more than once clutched his lower back, it was too close to call.

Morgan certainly was not pre-empting the decision when he said: "He had a back spasm yesterday ... if a fitness test is required in the morning he will go through that.

"He has not had it before, so it was a bit unexpected.

"It is one of those things that might cure itself after another night's sleep, a rub and a stretch."

Plan B for Alex Hales' opening partner appears to centre on either Root or all-rounder Moeen Ali - who did the job during last year's failed World Cup campaign.

"We have a number of options," said Morgan.

"We have had Moeen Ali bat at the top of the order and we have Joe Root who could slip up the order.

"We have James Taylor who could come in and bat at three, so certainly there is a number of options we have."