Initially, the Australian strike bowler believed the pain in his left side stemmed from a strained muscle but he now fears he has torn an abdominal muscle.

McGrath, who has been in the wars with injury since returning from the Ashes tour of England last year, has withdrawn from the remainder of the international one-day series against New Zealand and South Africa.

Australia cannot be beaten in the three-match Test series, having crushed South Africa by an innings and 21 runs in the second Test here on Monday. The first Test was drawn.

While McGrath remains optimistic about being fit for action in Adelaide, he concedes treatment for the injury will probably take at least three weeks.

``I went in for scans and I'm not too sure how they'll go but I think there's probably a tear in the abdominal muscles, which isn't good,'' he said. ``But I'm hoping I'll be fine for the Test. It just depends what treatment I'll need between now and then.''

McGrath said the injury was the result of straining his upper body to take weight off a groin injury during a one-day match in Adelaide on Nov 11. ``I had the nerve trouble a bit lower so this muscle was doing the extra work,'' he said.