Australian fast bowler Mitchell Starc used reverse swing to devastating effect Monday, claiming three wickets as Pakistan struggled to 100-7 on the third day of the second Test in Karachi.

At the tea break skipper Babar Azam was on a fighting 29 and Hasan Ali yet to score as the home team chase 257 to avoid the follow-on with just three wickets intact.

Australia had declared their first innings on a mammoth 556-9.

Starc, who went wicketless in the drawn first Test in Rawalpindi last week, had Azhar Ali (14), Fawad Alam (nought) and Sajid Khan (five) out as Pakistan lost six wickets in the space of 62 runs in the session.

It all went wrong in the first over after lunch when Imam-ul-Haq -- who scored twin centuries in the opening Test -- fell to a rash shot for 20, lofting spinner Nathan Lyon to skipper Pat Cummins at mid-on.

Starc then had Azhar caught in the slips and next ball trapped Alam with a sharp in-dipping yorker. He almost had a hat-trick when Mohammad Rizwan was beaten neck and crop, but didn't edge.

Rizwan didn't last long, however, as he edged Cummins for wicketkeeper Alex Carey to take a simple catch on six, and next over Cameroon Green trapped Faheem Ashraf leg before for four.

Starc then had Sajid caught behind, taking 3-24 in 10 incisive overs as wickets fell in a cluster, in contrast to Australia's run-spree on a low bounce National Cricket Stadium pitch.

Earlier, opener Abdullah Shafique was run out for 13 as Pakistan faltered at the start.

Shafique, who hit a brilliant unbeaten century in Rawalpindi, had made 13 of Pakistan's 26-run opening stand when he failed to beat debutant Mitchell Swepson's direct throw from point after being called for a sharp single.

Australia had resumed the third day on 505-8 as Cummins opted to bat on.

Pakistan struck with the second ball of the day when fast bowler Shaheen Shah Afridi had Starc caught at cover by Azhar without adding to his overnight 28.

But that was all the success they had as Cummins, who finished on 34 not out, added a brisk 51 for the ninth wicket with Swepson (15) before declaring.

All-rounder Ashraf was the pick of the attack with his 2-55 from 21 overs, while off-spinner Sajid Khan took 2-167 from 57 overs.

Australia are on their first tour of Pakistan since 1998, having previously refused to visit because of security reasons.

SCOREBOARD:

AUSTRALIA 1ST INNINGS (OVERNIGHT 505-8)

D. Warner c Rizwan b Ashraf            36

Usman Khawaja b Sajid                 160 

M. Labuschagne run out                  0

S. Smith c Ashraf b Hasan              72

N. Lyon b Ashraf                       38

T. Head lbw b Sajid                    23

C. Green b Nauman                      28

A. Carey b Azam                        93

M. Starc c Azhar b Shaheen             28

P. Cummins not out                     34

M. Swepson not out                     15

Extras (b10, lb7, nb7, w5)             29

Total: (for nine wkts dec; 189 overs) 556

Fall of wickets: 1-82 (Warner), 2-91 (Labuschagne), 3-250 (Smith), 4-304 (Lyon), 5-347 (Head), 6-360 (Khawaja), 7-405 (Green), 8-503 (Carey), 9-505 (Starc)

Bowling: Shaheen 32-8-95-1 (nb4), Hasan 25-7-71-1 (nb2), Ashraf 21-4-55-2, Sajid 57-10-167-2 (w3), Nauman 48-6-134-1 (w2), Azam 4-0-7-1, Azhar 2-0-10-0 (nb1) 

Pakistan: Imam-ul-Haq, Abdullah Shafique, Azhar Ali, Babar Azam, Fawad Alam, Mohammad Rizwan, Faheem Ashraf, Hasan Ali, Nauman Ali, Shaheen Shah Afridi, Sajid Khan 

Toss: Australia

Umpires: Aleem Dar (PAK) and Ahsan Raza (PAK) 

Tv umpire: Rashid Riaz (PAK) 

Match referee: Ranjan Madugalle (SRI)