Playing in his first match of the tournament since the outing against West Indies in their opener, Sohail smashed nine fours and three sixes to help the cause of Pakistan, who are struggling to stay alive in the semifinal race.
He added 81 runs for the fourth wicket with Babar Azam (69 off 80 balls), but it was his brisk 71-run partnership with Imad Wasim (23 off 15 balls) that gave Pakistan innings the impetus it needed in this must-win game.
The last 10 overs yielded 91 runs, with Sohail doing most of the damage.
Batting first, Pakistan were off to their most convincing start in the tournament, with the opening duo of Fakhar Zaman and Imam-ul-Haq adding 81 runs in just under 15 overs. They struck half-a-dozen boundaries each during their stay in the middle but fell for an identical 44.
Babar Azam oozed class as he worked the ball around for singles and twos in between seven boundaries, but the innings that propped up Pakistan was the one played by Sohail.
South Africa had their first breakthrough when Imran Tahir (2/41) had Fakhar scooping one to the safe hands of Hashim Amla at first slip.
Tahir went past Allan Donald as the highest wicket-taker for the Proteas at World Cups with 38 wickets when he produced an excellent one-handed catch to dismiss Imam.
Mohammad Hafeez hit a six during his 20 but could not translate his start into a substantial knock thanks to Aiden Markram, who had the batsman trapped in front of the wicket.