South Africa will hand a pre-play fitness check to Dale Steyn as they look to reverse the tide of the first Test against England in Durban.
Steyn, the home side's most potent bowler, twice left the field before completing an over on day three citing pain in his right shoulder.
Scans did not show any muscle tear but, after one risky attempt to return him to the attack lasted just three deliveries, South Africa will be wary of aggravating the 32-year-old's body on what could soon be a lost cause.
He has only just returned from a six-week lay-off with groin trouble and it is thought that spell on the sidelines may have be a contributing factor.
England resume on 172 for three - a lead of 261 - with Joe Root and James Taylor at the crease, knowing one good session should be enough to take the game away from the hosts.
South Africa team manager Dr Mohammed Moosajee accompanied Steyn to hospital on Monday, and said: "He found the pain and discomfort troublesome but the results of the scans are inconclusive.
"There are no tears to the muscles, tendons or ligaments and we are treating it as a shoulder spasm or stiffness for now.
"We will probably trial him in the warm-up and hopefully if he gets through that we may allow him to bowl. Further assessment will be done after the first Test before we make a call on Cape Town."
England are already in a similar position with their own champion seamer James Anderson, who was ruled out of the series opener with a calf strain and is striving to be fit for the New Year Test.
Moeen Ali, meanwhile, is shaping up to be a central figure for England as they press for victory over the next two days.
He is waiting to bat if needed at number eight, where he scored decisive late runs in the Ashes, and is sure to get through plenty of bowling in the fourth innings.
He took six wickets on the final day to finish off South Africa A in the recent warm-up match and four on a wearing deck in the Proteas' first innings at Kingsmead.
But he remains modest about his potential impact.
"We will have to play well for the next two days to hopefully get the win," he said.
"Hopefully wickets will come but I'll try not to get too carried away with the spin.
"I'm not going to stand here and say I'm a world-class bowler or anything like that. It's mainly being patient and learning about the art of bowling spin.
''It's tough but I feel like I'm learning quite a lot."