Du Plessis’ 55 and 64 from Duminy underpinned a strong total of 259 for eight after AB de Villiers won the toss at Abu Dhabi.
Leg-spinner Tahir then returned 4-53 as Pakistan were dismissed with 5.3 overs unused in the third of five one-day internationals between the sides.
The Proteas were without Graeme Smith, ruled out of the rest of the series with concussion, but his replacement Quinton de Kock made 40 from the top of the order.
De Plessis and Duminy followed up de Kock’s good work, allowing de Villiers and David Miller to score freely before a late flurry of wickets. Mohammad Irfan was Pakistan’s most prolific bowler with 3-46.
Ahmed Shehzad’s 24-ball 32 started the chase well but he was caught at slip off Dale Steyn in the over before Morne Morkel had Shezad’s opening partner Mohammad Hafeez held in the off side.
Pakistan again lost two wickets in two overs when Lonwabo Tsotsobe snared Umar Amin caught behind on review and Tahir trapped captain Misbah-ul-Haq in front. Three wickets in four mid-innings overs, including two for Tahir, did for Pakistan despite late resistance from Sohail Tanvir and Wahab Riaz.