Duminy scored 64 and added an invaluable 70 for the fourth wicket with AB de Villiers (34) to turn South Africa's good start to a challenging total after they won the toss and decided to bat.
The 29-year-old left-hander hit three boundaraies off 89 balls and gave the innings the much needed impetus with a 66-run fifth wicket stand with David Miller (34), adding 65 in the last ten overs.
Du Plessis had steadied the innings during his 60-ball knock, adding 77 for the second wicket with Quinton de Kock who made 40 from 57 balls.
Graeme Smith -- ruled out of the series due to post-concussion after being hit on the head by a bouncer in the second Test in Dubai -- Colin Ingram and Wayne Parnell were left out.
More From This Section
Pakistan brought in Asad Shafiq in place of flopped opener Nasir Jamshed.
Leg-spinner Shahid Afridi (2-40) derailed the innings by dismissing De Kock and Du Plessis in quick succession, both stumped.
Du Plessis hit eight boundaries while De Kock crossed the boundary five times.
When South Africa looked set for a big total with Duminy and Miller at the crease, off-spinner Saeed Ajmal (2-38) dismissed Miller before Sohail Tanvir had Duminy caught at mid-wicket off a misjudged pull.
Irfan then dismissed Ryan McLaren (13) and Dale Steyn (two) to halt South Africa's progress.
The five-match series is tied at 1-1.