The Springboks close the tournament against the Pumas back home in Durban and by then will know what they have to do to win their second title in the Rugby Championship era.
They set up their chance with a first-half display of relentless forward might that made the Pumas lose their heads, then weathered a Pumas comeback, and finished brilliantly with two tries in the last five minutes to recapture the bonus point.
South Africa scored three tries to none in a first half that was constantly stopped, particularly for offsides.
The first try was a penalty try, to no surprise. South Africa attacked the short side off a lineout maul, Pumas first-five Santiago Carreras appeared to stop it, but then came from offside to hold up Springboks halfback Jaden Hendrikse over the line. Carreras conceded the penalty try and was sin-binned.
While he was away, the Springboks crossed twice more.
After the Boks went for three consecutive attacking lineouts instead of penalty kicks, Hendrikse wasn't considered a threat by the Pumas and allowed to stroll to the tryline. Then the Springboks produced a textbook lineout maul that carried hooker Malcolm Marx over the line to lead 22-6 after 32 minutes.
Argentina were playing with scraps, and wasting what little they had. Warned repeatedly by New Zealand referee James Doleman, who penalised the Pumas 12 times in the half, the home side lost a second player to the sin bin, halfback Gonzalo Bertranou, and almost conceded another try on halftime. But Lood de Jager's score was rubbed out by a knock-on.
Down by 16, the Pumas threw caution to the wind in the new half with offloads galore, and finally roused their frustrated fans in their last home match of the year.
With the heat turned back on a somewhat complacent South Africa, fullback Willie le Roux was sin-binned.
The Pumas received another fillip when former captain Agustin Creevy came on and became Argentina's most capped player in his 95th test.
With the man advantage, Argentina earned a penalty try, and with a two-man advantage scored another try to close the gap to 22-20 with 10 minutes to go.
The Pumas' penalty try came when replacement halfback Tomas Cubelli was held up but twisted around the neck and prevented from reaching out for the line by Kwagga Smith, who was sin-binned.
Then another replacement back, Matias Moroni, took an inside pass from flanker Marcos Kremer to break clear, slip Canan Moodie and carry the other Boks wing, Makazole Mapimpi, between the posts.
But the razzle-dazzle effort to overcome the 16-point deficit exhausted the Pumas, and the Springboks finished strong.
They worked the phases to get centre Damian de Allende over in the 75th, and worked an overlap for Mapimpi, whose inside pass to Marx was juggled but secured. His second try restored the bonus point the Springboks dearly wanted with a victory to set up an intriguing last round.
South Africa 36 (Malcolm Marx 2, Jaden Hendrikse, Damian de Allende tries, penalty try; Damian Willemse con, pen, Frans Steyn 2 cons)
Argentina 20 (Matias Moroni try, penalty try; Emiliano Boffelli con, 2 pens)
HT: 22-6
