Pakistan 148 and 192 for 2 (Babar 102*, Shafique 71*, Green 1-15) need another 314 runs to beat Australia 556 for 9 dec and 97 for 2 dec (Khawaja 44*, Labuschagne 44, Afridi 1-21)
Captain Babar Azam hit a stirring century and combined brilliantly with opener Abdullah Shafique to defy Australia's push for victory, as Pakistan's stubborn resistance left the second Test intriguingly poised ahead of the fifth day.
Needing to survive 172 overs, Pakistan were 192 for 2 at stumps on day four with Babar on 102* and Shafique unbeaten on 71. Pakistan still need another 314 runs for an unlikely record-setting victory, but have made Australia toil through 82 overs in a far cry to their toothless first innings of 148.
Having played a lone hand in Pakistan's first innings, Babar notched his sixth Test century - his first in two years - with a sweep over fine leg just before stumps. He let out a roar amid jubilation from a sparse crowd after which he and Shafique survived two overs against the second new ball and made it through to the close.
After Pakistan were reeling at 21 for 2, Babar and Shafique combined for a rousing partnership of 171 to thwart Australia's attack and provide a flicker of hope for the hosts, who had been hapless for most of this match previously.
With oppressive conditions in Karachi throughout the match, the pitch is marked by widening cracks in a contrast to the docile Rawalpindi deck amid a stale first Test draw. There was sharp turn and variable bounce evident, particularly earlier in Pakistan's innings, but the pitch did not appear to deteriorate as the day wore on.
Babar and Shafique were mostly untroubled, and blunted the bowling through almost two sessions in somewhat of a throwback to Rawalpindi when Australia claimed just four wickets in 239 overs. The visitors were left to rue two early chances against first-Test centurion Shafique, who started slowly before finding fluency after being joined at the crease by a positive Babar.
Shafique was dropped on 20 by Steven Smith, whose struggles at slip continued when he spilt a straightforward chance. Even though he was standing close, Smith will feel aggrieved at his fumble, having been below par with his slip catching during the Ashes too.
After Pakistan endured two horror run-outs in the first innings, Shafique was almost dismissed short of his crease for the second time in the Test only for Cameron Green's throw from mid-on to miss the stumps. Australia weren't able to penetrate with Mitchell Starc either, who had starred with 3 for 29 on day three, as he was unable to conjure the type of menacing reverse swing that had lit a fuse under the Test 24 hours earlier.
Mitchell Swepson, Australia's first specialist legspinner since Bryce McGain in 2009, was loose in Pakistan's first innings but looked more comfortable the second time around, and occasionally conjured sharp fizz much like his hero Shane Warne, who mentored him.
Late in the day, he was entrusted by Pat Cummins to break the partnership as Swepson bowled from around the wicket aiming for the rough patch outside the leg stump of the two right-hand batters with four catchers around the bat, but a confident Babar continually padded the ball away to frustrate Australia.
Cummins, who has seemingly made all the right moves in this Test, turned to himself before stumps and conjured sharp swing with the second new ball but to no avail.
It has been a brave effort from Pakistan amid some expectation that Australia could wrap up victory inside four days after dominating since the get go. Things appeared to be going to plan for Australia when Pakistan started their daunting task disastrously before lunch, as opener Imam-ul-Haq was plumb lbw for 1 to a sliding Nathan Lyon delivery. It completed a miserable match for Imam, who had fallen loosely to Lyon on 20 in the first innings after twin centuries in Rawalpindi.
Pakistan were intent on mere survival, with Azhar Ali stonewalling for 6 off 54 balls, but he fell lbw in the second over after lunch while ducking into a short Green delivery that skidded low. It was reminiscent of Glenn McGrath's famous wicket against Sachin Tendulkar in Adelaide in 1999, but replays showed Azhar had gloved the ball before it hit his body as he trudged off ruing not calling for a review.
Earlier in the day, Australia gave themselves plenty of time to take a series lead after Cummins declared their second innings 25 minutes into day four with the score at 97, their third-lowest declaration in Tests. The only other time Australia had declared both their innings in a Test in Asia was against India during the famous tied Test in Chennai in 1986.
Usman Khawaja, who had hit three centuries in six previous innings since his recall, finished on 44* as the declaration cut short his chance of getting twin hundreds in the same Test after he achieved the feat in Sydney on his recall. However, the declaration turned out to be a wise call, with Australia coming up against stiff resistance from Pakistan, who have lifted off the canvas.
