Batting first Windies posted 222/8. In reply Proteas chased down the target with three wickets in hand and 6.1 overs to spare.
Rilee Rossouw was the saviour of South Africa as he anchored the run-chase and saw the side home with a timely knock after his side stuttered in the middle.
Proteas, chasing a decent 223-run target, started off disastrously losing Pieter Malan even before a run had been scored. Windies seamer Jason Dawes' full length ball rapped Malan on his back foot just below the flap on off and middle stump line.
It was perhaps no more than the Windies" bowlers expected having made the batting lineups of Australia and Sri Lanka look ordinary in the warm-up matches last week.
But then Jon-Jon Smuts and Reeza Hendricks added 74 for the second wicket. The duo scored runs at brisk pace.
Smuts in particular was severe on Windies new-ball bowlers who erred in length too often. He hit Windies pacemen all round the park. His back-foot punch off Grant sailed like a bullet out of AK-52 to covers boundary. Smuts hit nine boundaries in his run a ball 45 before Perumal halted his surge for a plumb LBW decision.
Right-arm military medium pacer Perumal struck again to send Hendricks packing for a caught and bowled offer. At this point of time South Africa looked comfortable at 92/3 in 19 overs with still a quality batters to follow.
However Windies leggie Shamarah Brookes had other plans. He ran thro' the SA middle-order to reduce 'em to 179/7 despite Vandiar's (34 off 29) little cameo. Rossouw found an able ally in Roy Adams to ensure his side home.
Earlier, captain Wayne Parnell (3/37) and Jacobus Pienaar (3/36) bowled well to restrict the West Indies to 222 for 8. The hard-hitting opener Kieran Powell (53) and later Devon Thomas (53) were the chief run-getters for the West Indies.
Larry Gomes' team must win its remaining two games in the group if it is to nurture any hope of making it to the Super League.
Proteas will take on India in what could be a Group-B decider at the same venue 'morrow.
With India having beaten Papua New Guinea in the opening fixture of the ICC U/19 Cricket World Cup both teams are now on one win from one and so whichever side wins tomorrow is assured of progressing through to the Super League stage of the game.
Windies 222/8 in 50 overs (Karren Powell 53, D Thomas 53, S Thomas 41; Pienaar 3 for 36, Parnell 3 for 37) lost to South Africa 223/7 in 43.5 overs (Rossouw 58 n o, Jon-Jon Smuts 45, Raeez Hendricks 36, Jonathan Vandier 34, Roy Adams 26 n o; Shamarah Brookes 4 for 39, Perumal 2 for 35) by three wickets.