Guptill (103 from 134 balls) reached his ninth ODI century as he hit the winning runs to complete the emphatic victory for his side.
Set a paltry 205 to win after the Proteas showed little with the bat, the Kiwi openers Guptil and Tom Latham (64) set about compiling the highest-ever first wicket partnership for New Zealand against South Africa in ODIs. Their opening stand of 126 broke the back of the total, and though South Africa opened up an end, as they collected two wickets, the result was never in doubt.
The visitors reached their target in 44.3 overs with Guptil and George Worker (20) unbeaten in the middle.
The third ODI is scheduled for Kingsmead in Durban, on Wednesday.
Farhaan Behardien top-scored with a fighting knock of 70 from 87 balls and was a shining light for the Proteas in an otherwise dismal display with the willow.
The hosts were in heaps of trouble early at 20 for two, with openers Hashim Amla (8) and Morne van Wyk (2) both departing cheaply.
After two crisp boundaries whipped off his legs, Amla chased a wide delivery from Adam Milne and his miscued shot was taken at third man by Grant Elliot. Next Van Wyk fell, attempting to cut a ball too close to his body as the ball made a mess of his stumps.
A 48-run third wicket partnership from 68 balls was stopped in its tracks when Rilee Rossouw (39) miscued a shot over the infield, as his shot presented a comfortable catch for Williamson off the bowling of Doug Bracewell as South Africa fell to 68 for three in the 16th over.
AB de Villiers departed for 31 off 45 as he was outfoxed by an Elliot delivery which kicked off the surface and Guptill took a simple catch at short mid-wicket to leave the hosts reeling on 107 for five after 23.4 overs.
It continued to be a struggle for the Proteas throughout the innings as the batsmen battled to put partnerships together, demonstrated by the seventh wicket stand of 54 between Behardien and Vernon Philander (30 not out).
Pick of the bowlers for New Zealand was Bracewell who claimed 3/31 from his 10 overs.
– African News Agency (ANA).A Martin Guptill ton has led NZ to an emphatic victory over SA in the second ODI in Potch.
Original source: NZ thump Proteas in Potch