The Proteas ensured that they went into the last day with one foot on the podium and eventually romped to a resounding victory.
The two West Indies senior batsmen, Marlon Samuels and Shivnarine Chanderpaul, raised hopes of their setting the Castle Lager Proteas a challenging target in the third Sunfoil Test match at Newlands on Monday but then Samuels got caught on the boundary to spark a catastrophic collapse that saw the visitors lose their last 6 wickets for 33 runs.
The result was that they were only able to set the Proteas a target of 124 and, although they dismissed Alviro Petersen in the three overs that were possible before the close, the odds will not be in their favour going into Tuesday's final day of the series. The Proteas are 9/1 and need a further 115 to tie up the series 2-0 and ensure they remain the world's No. 1 team for the foreseeable future.
During the lower-order collapse Steyn (3/75 in 23.5 overs) took two wickets off successive balls while Harmer (4/82 in 24) took two wickets in 3 balls as the last five wickets fell for just 13 runs. Harmer has had an outstanding debut, finishing with 7 wickets in the match. He has done everything asked of him by providing the control to assist the seamers in the first innings and becoming the leading wicket-taker in the second.
With South Africa's next two scheduled Test series on the sub-continent against Bangladesh and India he can look forward to further opportunities.
As well as Steyn and Harmer performed the best bowling of the day may well have come from Morne Morkel who took 2/18 in 14 overs to continue the excellent form he has shown throughout the series.
The West Indies resumed on a rain-shortened day consisting of a single 53 overs session with an overall deficit of 4 runs and 8 wickets in hand. Leon Johnson did not last long against Morkel to bring to an end a stand of 68 that had stabilised the innings. Samuels, who launched a calculated attack on Harmer until he went to the well once too often to fall to a running catch on the long-on boundary by Dean Elgar, and Chanderpaul looked like batting the innings into the final day.
They added 87 for the fourth wicket before Samuels perished (74 off 150 balls, 9 fours and 2 sixes) and Chanderpaul then had to watch one partner after another depart in quick succession. He was eventually last man out (50 off 113 balls, 5 fours), run out by a direct hit by Temba Bavuma.
In return on the last day the Castle Lager Proteas confirmed themselves as the No. 1 team on the ICC rankings when they beat the West Indies by eight wickets with two sessions to spare in the final Sunfoil Test match at Newlands on Tuesday.
It meant that they won the three-match series 2-0 and also retained the Sir Viv Richards Trophy for competition between the two countries. They will also now top the rankings at the cut-off point in April as Test cricket takes a break while the ICC World Cup takes place in Australasia.
Their top ranking earns them a bonus of $US 500 000.
Needing only a further 115 runs for victory with nine wickets in hand the Proteas were put under early pressure by Suleiman Benn who gained both turn and bounce from the fifth day surface. There was also tight bowling at the other end from Jerome Taylor and it took Dean Elgar and Faf du Plessis the best part of the opening half hour to get the scoreboard moving.
Du Plessis did not last long, becoming Benn's second victim on his way to innings figures of 2/24 in 17 overs. However, Elgar (60 not out, 103 balls, 7 fours and a six) and Hashim Amla (38 not out, 49 balls, 6 fours) then finished the job with an unbroken partnership of 73.
AB de Villiers was named Sunfoil Man of the Match for his first innings century while Amla, who scored a double century in the victory at SuperSport Park, was named Sunfoil Man of the Series.