Shehzad was unbeaten on 55 for his third fifty as Pakistan reached 116-1 at lunch on the fourth morning, stretching their lead to 267 and further press their claim for a 1-0 lead in the two-match series.
With Shehzad was first innings century-maker Younis Khan, on 26, after Pakistan had resumed on 38-0.
Pakistan had taken a big 151-run lead in the first innings after scoring 454 and dismissing Australia for 303.
Australia had looked to stop the flow of runs and had a good chance of breaking through early, but Brad Haddin failed to grasp a low catch off spinner Steve O'Keefe when Shehzad edged one on to the pads which popped out of wicketkeeper's hands.
But O'Keefe got his reward when he induced an edge off Ali straight into the hands of Haddin with the score at 71. Ali made 30.
Shehzad reached his fifty off 128 balls with a single and has so far hit four fours.
Younis hit a six off Lyon to continue the solid form he showed in the first innings when he made 106.

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