"I felt a whole lot better than I did two days ago. I rested up for the first game, had a bit of a rest day yesterday and came through training this morning pretty well, so I was able to play.''
Guptill finished unbeaten on 101 as his boundary off the last ball helped New Zealand level the three-match series at 1-1.
He became only the second New Zealander to have made an international Twenty20 century, following the path of the side's new skipper, Brendon McCullum.
Earlier in the day, South Africa posted 165-5 from 19 overs after the game was reduced due to the failure of one of the stadium's floodlight towers.
Proteas captain Faf du Plessis top-scored for the home side with 63 and he was well-supported by opener Henry Davids who made 55.
Doug Bracewell was the most successful of the Kiwi bowlers as he picked up 3-33, while Nathan McCullum was miserly as his four overs only conceded 23 runs.
The vagaries of the Duckworth-Lewis Method were again in full force as it dictated that New Zealand then had to chase 169 for victory.
Guptill and Rob Nicol began well as the pair put on 76 for the first wicket before Nicol was sent on his way for 25 and Brendon McCullum then added a brisk 17 to keep the score ticking over.
McCullum's departure brought Colin Munro to the crease who showed useful composure to work the ball around in the final overs and the last act was left to Guptill.
With four required for victory, du Plessis bizarrely pulled his field in on the off-side - with the exception of a man at long-off - and Guptill gleefully accepted Rory Kleinveldt's over-pitched delivery to hit it over cover for four.
As the ball reached the rope Guptill was visibly fired-up as he screamed to the New Zealand dug-out, and you could see how much the victory meant to the 26-year-old.
McCullum said he was proud of his side's resolve to come back from such a heavy defeat in game one, where they were destroyed by eight wickets.
"The boys are incredibly happy at the moment and obviously very proud of the way Martin stepped up under some big pressure there.''
The series-deciding game will be played at Port Elizabeth in the early hours of Thursday morning (NZT).