Sri Lanka's Kusal Mendis (right) watches his catch out by Pakistan's Babar Azam in Karachi on Sunday. Asif Hassan/AFP

Pakistan grabbed three early wickets pressing for a series-win Sunday in the second Test against Sri Lanka after home captain Azhar Ali hit a return-to-form century in Karachi.

Set a daunting 476-run target to win, Sri Lanka lost skipper Dimuth Karunaratne for 16, Kusal Mendis for a duck and Angelo Matthews for 19 as Pakistan's pace attack spat fire on a lifeless National stadium pitch.

Pakistani batsman Babar Azam acknowledges his 100 against Sri Lanka during the second Test in Karachi. AP

At tea, Sri Lanka were on the back foot at 86-3 with opener Oshada Fernando unbeaten on a fighting 47 and Dinesh Chandimal on two.

Sri Lanka need another 390 runs with seven wickets remaining.

Pace spearhead Mohammad Abbas had Karunaratne caught behind in the tenth over of the innings before 16-year-old paceman Naseem Shah had Mendis snapped up in the slips in the next over.

It became 70 for three when Mathews, Sri Lanka's most experienced batsman, edged a short delivery from Shaheen Shah Afridi to wicketkeeper Mohammad Rizwan.

Pakistan declared their second innings at 555 for three at lunch after Azhar and Babar Azam (100 not out) joined openers Abid Ali (174) and Shan Masood (135) in the run-fest.

It was only the second time in a Test innings that the top four had all scored centuries after openers Abid Ali (174) and Shan Masood (135) grabbed theirs on Saturday. India against Bangladesh at Dhaka in 2007 was the other occasion.

Pakistani batsman Babar Azam plays against Sri Lanka during the second Test in Karachi. Fareed Khan/AP

Azhar, who had a lean 2019 with no fifty, completed his 16th century before he was stumped off spinner Lasith Embuldeniya after 11 boundaries in his 157-ball innings.

Azam's fourth century -- and third in the last four Tests -- had seven boundaries and a six off 131 balls.

Earlier, when Pakistan resumed at 395 for two, Azhar smashed three boundaries off pace bowler Vishwa Fernando and then took two runs to complete his first century in 12 months.

Azhar added 148 for the third wicket with Azam as Sri Lankan bowlers toiled hard for wickets. Lahiru Kumara finished with two for 139.

The Test series is the first in Pakistan since the attacks on the Sri Lankan team bus in 2009 suspended international cricket in the country.