The 31-year-old former Leeds United and Liverpool forward was named in Pim Verbeek's preliminary squad last week but is still struggling with a problem picked up while playing for his Turkish club Galatasaray.
"I'm doing all the right things. Nothing's pulling up short. We're working on a schedule and it's all going on track," he told the Australian Associated Press (AAP) on Monday.
"It's important that I am (at 100 percent)... not only for myself, but for my team as well," he added. "If I can't be 100 percent, I'm letting myself down and my team and my country as well."
Kewell, who may be asked to play as a lone forward against the Germans in Durban on June 13, will almost certainly miss this week's warm-up against New Zealand.
Australia, who reached the first knockout stage at the last World Cup in Germany, also face Serbia and Ghana in first round Group D at the June 11-July 11 tournament.
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