South Africa 264 for 6 (Klaasen 119*, Joseph 3-50) beat West Indies 260 for 9 (King 72, Fortuin 2-46) by four wickets
Potchefstroom: Heinrich Klaasen hit a 54-ball century as South Africa raced to a four-wicket win over West Indies in the third one-day international in Potchefstroom.
Chasing 261 to win, Klaasen came to the crease at 80-4 but smacked an unbeaten 119 as South Africa won in 29.3 overs.
It is the fastest any side has chased more than 250 in a men's ODI.
West Indies were bowled out for 260 with Brandon King making 72 and left-arm seamer Marco Jansen and spinner Bjorn Fortuin both taking 2-46.
The series finishes 1-1 after the opening ODI was washed out.
South Africa will now face the Netherlands on 31 March and 2 April, with the games deciding who earns the eighth and final automatic qualification spot for October's 50-over World Cup.
West Indies currently occupy eighth with 88 points, while South Africa are ninth with 78 points.
Ten points are awarded for a win but teams level on points are split by wins before net run-rate, meaning South Africa must win at least one of the games against the Netherlands and avoid defeat in the other to climb above West Indies.
Sri Lanka, who are currently 10th with 77 points and play a three-match series in New Zealand between 25-31 March, could qualify if South Africa slip up.
The eighth-place team will join hosts India, defending champions England, Australia, New Zealand, Bangladesh, Pakistan and Afghanistan in the tournament that will take place in October and November.
The teams that miss out will go into the World Cup Qualifier tournament which is set to take place in Zimbabwe between 18 June and 9 July.
Ireland and Scotland are already in the qualifier alongside Zimbabwe, the Netherlands, Nepal and Oman. Two teams from that tournament will qualify for the full competition.