Australian captain Aaron Finch suffered a finger injury at training on Friday after being struck on the glove by fast bowler Mitchell Starc.
Batting in the opening over of a centre-wicket practice session at the WACA, Starc hit Finch on the glove with a searing delivery.
Finch seemed visibly distressed after the blow, throwing his bat away, taking his glove off to examine his injured right index finger.
The 31-year-old appeared to be about to resume batting, but he then left the ground after consulting the team doctor and physiotherapist.
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Minutes later, Finch left Australia's dressing room with his finger in a cup of ice.
The news is a blow just two days out from the first One Day International against South Africa at Perth's Optus Stadium on Sunday.
It's unclear whether Finch's injury will keep him out of the match, which is due to be his first as Australia's full-time ODI skipper.
If he is ruled out, Josh Hazlewood or Alex Carey, who have been named vice-captains, will take over as skipper.
Meantime, Starc had been dealing with a hamstring problem but he got through the session at the WACA unscathed.
It will be the first of three ODIs for Australia against the Proteas, with game two to be played at Adelaide Oval on Friday, November 9, and game three on Sunday, November 11 in Hobart.
The two sides will then square off in a lone Twenty20 International on the Gold Coast on Saturday, November 17.
Australia squad: Aaron Finch (c), Josh Hazlewood (vc), Alex Carey (vc), Ashton Agar, Nathan Coulter-Nile, Pat Cummins, Travis Head, Chris Lynn, Shaun Marsh, Glenn Maxwell, D'Arcy Short, Marcus Stoinis, Mitchell Starc, Adam Zampa.
South Africa squad: Faf du Plessis (c), Farhaan Behardien, Quinton de Kock, Reeza Hendricks, Imran Tahir, Heinrich Klaasen, Aiden Markram, David Miller, Chris Morris, Lungi Ngidi, Andile Phehlukwayo, Dwaine Pretorius, Kagiso Rabada, Tabraiz Shamsi, Dale Steyn.