As it happened: Pumas v Boks
Trailing by 12 points after 56 minutes, a Springbok side given a torrid time in the forward exchanges for much of the match clawed back to snatch victory.
The scrums were a particular problem for the two-time world champions with the Argentines shoving them back metres several times.
Success on a hot, clear afternoon in north-western Argentina lifted leaders South Africa to eight points on the southern hemisphere championship table, one above title-holders New Zealand.
Defeat left the often dominant Pumas wondering what they must do to end a 14-match win-less run since debuting in the competition two seasons ago.
Springboks centre and skipper Jean de Villiers cut a relieved figure after the close call on a day when New Zealand gave a masterclass in a 51-20 drubbing of Australia.
"We stuck to our guns and won," he sighed. "Argentina are getting better and better -- and played unbelievably well today.
"They created many challenges for us. And I am just happy that we beat them a second time within eight days.
"There were problems at scrum time, but when it really counted, the pack did well.
"This team is a work in progress. And there are lots of errors to be corrected.
"Youngsters like Damian de Allende and Handre Pollard are on a steep learning curve and they will get better and better."
A stong wind favoured Argentina in the opening half and early pressure from the hosts culminated in fly-half Nicolas Sanchez kicking a close-range penalty.
Tentative South Africa battled to get inside the Pumas' half and were lucky when a marginal forward-pass call prevented the home side scoring a try.
But the visitors levelled when fly-half Pollard slotted a penalty, keeping his nerve amid whistling and booing from a fiercely partisan crowd.
A snap drop goal by veteran centre Juan Martin Hernandez restored the Pumas' lead only for Pollard to level again through another penalty.
Referee Steve Walsh gave the teams a water break midway through the half and soon after Argentina led for the third time via a try from big left-wing Manuel Montero.
Sheer power took the Pumas close and when they could not barge over, the ball was passed quickly across the field to set up a score in the corner which Sanchez converted.
Pollard narrowed the gap to four points with his third penalty as the visitors struggled to stamp any authority on a scrappy opening half.
Springboks scrum-half Ruan Pienaar then temporarily switched codes to football as he created a barely deserved try for left-wing Bryan Habana.
Pienaar booted the ball along the ground three times as a counter-attack caught the home team napping and Habana raced up to touch down. Pollard converted for a 16-13 half-time advantage.
Undaunted, the Pumas made a great start to the second half and replacement scrum-half Tomas Cubelli and full-back Joaquin Tuculet scored tries within four minutes.
Cubelli evaded two weak tackles after a scrum to dart over and Tuculet held off De Allende and De Villiers to dot down in the corner. Sanchez converted the first try.
The fly-half also succeeded with a close-range penalty and Argentina were 28-16 ahead and sensing victory.
South Africa needed a try quickly and right-wing Cornal Hendricks obliged, going over in the corner.
Replacement Steyn converted and there were just five points between the teams as they took a second water break.
The Springboks edged ahead for only the second time in the match when substitute flank Marcell Coetzee scored a try off a team favourite - the driving maul.
Steyn converted, South Africa led by two points, and the crowd went silenced.
Not for long, though, as centre Marcelo Bosch took a penalty from inside his half and a high kick went between the points to give Argentina a 31-30 advantage.
But regular Springboks match-winner Steyn had the final say, keeping calm under intense pressure to slot the match-winning penalty.
Scorers:
Argentina
Tries: Manuel Montero, Tomas Cubelli, Joaquin Tuculet
Conversions: Nicolas Sanchez (2)
Penalties: Sanchez (2), Marcelo Bosch
Drop goal: Juan Martin Hernandez
South Africa
Tries: Bryan Habana, Cornal Hendricks, Marcell Coetzee
Conversions: Handre Pollard, Morne Steyn (2)
Penalties: Pollard (3), Steyn
Argentina
15 Joaquin Tuculet, 14 Lucas Gonzalez Amorosino, 13 Marcelo Bosch, 12 Juan Martin Hernandez, 11 Manuel Montero, 10 Nicolas Sanchez, 9 Martin Landajo; 8 Juan Manuel Leguizamon, 7 Juan Martin Fernandez Lobbe, 6 Pablo Matera, 5 Thomas Lavanini, 4 Mariano Galarza, 3-Ramiro Herrera 2 Agustin Creevy (captain), 1 Marcos Ayerza
Substitutes: 16 Matias Cortese, 17 Bruno Postiglioni, 18 Nahuel Tetaz Chaparro, 19 Matias Alemanno, 20 Leonardo Senatore, 21 Tomas Cubelli, 22 Jeronimo de la Fuente, 23 Horacio Agulla
South Africa
15 Willie le Roux, 14 Cornal Hendricks, 13 Damian de Allende, 12 Jean de Villiers (captain), 11 Bryan Habana, 10 Handré Pollard, 9 Ruan Pienaar, 8 Duane Vermeulen, 7 Juan Smith, 6 Francois Louw, 5 Lood de Jager, 4 Eben Etzebeth, 3 Jannie du Plessis, 2 Bismarck du Plessis (vice-captain), 1 Gurthrö Steenkamp
Substitutes: 16 Adriaan Strauss, 17 Tendai Mtawarira, 18 Frans Malherbe, 19 Bakkies Botha, 20 Marcell Coetzee, 21 Francois Hougaard, 22 Morné Steyn, 23 Lwazi Mvovo