Islamabad-born Khawaja had fallen agonisingly short of a hundred in the opening Test in Rawalpindi where he top-scored for his side with a breezy 97.

The elegant left-hander made amends in Karachi where he shelved the reverse sweep and made an unbeaten 127, which included 13 fours and a six.

Smith made 72 before falling to Hasan Ali in the penultimate over of the day.

Nightwatchman Nathan Lyon had yet to open his account.

Earlier, Australia got off to a strong start after Pat Cummins elected to bat on a dry surface at the National Stadium which offered reverse swing as early as in the second session.

Khawaja looked in sublime touch and scored freely on the leg-side after being fed regularly on his pad.

His opening partner David Warner was less convincing, though, surviving a couple of LBW appeals and getting hit on his shoulder by a short delivery from Hasan.

Warner hit Sajid Khan for a six to bring up Australia's 50 in the 11th over.

Sajid conceded two more sixes in his fourth over, prompting Pakistan captain Babar Azam to take the spinner out of the attack.

Seamer Faheem Ashraf broke the 82-run opening stand with a back of a length delivery that Warner, who made 36, could only edge to Mohammad Rizwan behind the stumps.

Pakistan tasted a second success soon afterwards when Sajid ran out Marnus Labuschagne for a duck with a direct throw from mid-off.

Khawaja and Smith steadied the ship batting through the second session.

Khawaja took a single off Nauman Ali to bring up his third hundred in four Tests since his recall in January after more than two years' absence.

Pakistan spinners resorted to a negative line after tea, bowling way down the leg-side, to stem the run flow and the batsmen were happy to just pad them away.

Smith, who made 78 in Rawalpindi, once again failed to convert his fifty into a hundred and edged Hasan for Ashraf to take a low one-handed catch at second slip.

Leg-spinner Mitchell Swepson makes his Test debut, joining Lyon in Australia's two-man spin attack with Josh Hazlewood sitting out.

Teams

Pakistan: Babar Azam (captain), Imamul Haq, Abdullah Shafique, Azhar Ali, Fawad Alam, Mohammad Rizwan, Faheem Ashraf, Hasan Ali, Nauman Ali, Shaheen Shah Afridi, Sajid Khan

Australia: Pat Cummins (captain), David Warner, Usman Khawaja, Marnus Labuschagne, Steve Smith, Travis Head, Cameron Green, Alex Carey, Mitchell Starc, Mitchell Swepson, Nathan Lyon