After dismissing Pakistan for just 213, South Africa reached their target with a full eight overs to spare to take an unassailable 3-1 lead in the five-match series.
Kirsten, who had admitted he thought his one-day international career was over, scored an unbeaten 102 off 118 balls with nine fours to see South Africa home with Boeta Dippenaar, who made 47 not out.
The 35 year-old left-hander rode his luck at times, surviving a hugely confident appeal for lbw by Waqar Younis when he was on just two and dropped at cover by Shoaib Akhtar on 31.
The only South African wicket to fall was that of Herschelle Gibbs, bowled by a good delivery from Abdur Razzaq for a crisp 52 off 58 balls including six fours and a six.
Earlier, Yousuf Youhana held together a shaky Pakistani batting effort, top-scoring with 61 from 71 balls as the tourists lost wickets at regular intervals after winning the toss and making a brisk start.
That was provided by young wicketkeeper Kamran Akmal and Salim Elahi, but the slide began when Akmal was bowled for 31 by Makhaya Ntini with the score on 54.
Allan Donald and Jacques Kallis then struck in successive overs, Razzaq (5) and Elahi (26) both unquestionably lbw, and although Youhana and Inzamam-ul-Haq put on 52 for the fourth wicket, when Inzamam (34) was trapped lbw again by Robin Peterson, Pakistan were 129 for four.
None of the lower order provided Youhana with significant support, and the right hander was eventually eighth out after a misunderstanding with Saqlain Mushtaq, Jonty Rhodes swooping from point to throw down the stumps.
Mark Boucher’s catch to dismiss Younis Khan off Ntini was his 200th dismissal in his 136th one-day international, the second fastest to the milestone after Australia’s Adam Gilchrist.
The final match of the series is in Cape Town on Wednesday.
PAKISTAN
Kamran Akmal b Ntini 31
Salim Elahi lbw b Kallis 26
Abdur Razzaq lbw b Donald 5
Yousuf Youhana run out (Rhodes) 61
Inzamam-ul-Haq lbw b Peterson 34
Younis Khan c Boucher b Ntini 20
Faisal Iqbal run out 9
Shahid Afridi b Kallis 0
Saqlain Mushtaq b Pollock 6
Waqar Younis not out 13
Shaoib Akhtar c and b Pollock 0
EXTRAS (lb-1 w-7) 8
TOTAL (all out, 48.4 overs) 213
FALL OF WKTS: 1-54 2-65 3-67 4-129 5-166 6-183 7-190 8-196 9-213
BOWLING: Pollock 9.4-1-39-2 (w-1), Ntini 10-2-44-2 (w-2), Donald 8-0-35-1 (w-2), Kallis 9-0-35-2 (w-2), Peterson 7-0-29-1, Klusener 5-0-30-0
SOUTH AFRICA
H.Gibbs b Abdul Razzaq 52
G.Kirsten not out 102
B.Dippenaar not out 47
EXTRAS (lb-4 nb-5 w-4) 13
TOTAL (for one wicket, 42 overs) 214
FALL OF WKT: 1-111
DID NOT BAT: Jacques Kallis, Jonty Rhodes, Mark Boucher, Lance Klusener, Shaun Pollock (captain), Robin Peterson, Allan Donald, Makhaya Ntini.
BOWLING: Waqar Younis 8-0-47-0 (w-2) Shoaib Akhtar 7-0-35-0 (w-1) Saqlain Mushtaq 10-0-52-0, Abdul Razzaq 7-0-32-1 (nb-5, w-1) Shahid Afridi 10-0-44-0.
RESULT: South Africa won by nine wickets
FIRST MATCH: Durban - South Africa won by 132 runs
SECOND MATCH: Port Elizabeth - Pakistan won by 182 runs
THIRD MATCH: East London - South Africa won by 62 runs
FIFTH MATCH: Dec 18, Cape Town.—Reuters