A stellar first half performance on attack was enough to hand the SA 'A' side a 17-3 lead at the break, but heroic defence in the second 40 minutes held sway in the final result of the match.
The fact that the visitors started the second half playing against 13 men, with Faf de Klerk and Marco van Staden sin-binned, gave the tourists momentum and a converted try and penalty goal in return, but they were slowly put back on their heels by a green tackling machine.
The early action showed why many regarded this as the fourth test match on tour. The physicality of the hits and breakdowns from both sides were immense with every inch earned the hard way.
The first try-scoring opportunity came from a turn-over. Joseph Dweba (hooker) made early inroads with a good steal and on the counterattack Sbu Nkosi was ruled to have put a foot in touch when he went over in the corner after five minutes.
The Lions were offside in the lead-up though and Morné Steyn kicked the first points of the match for some reward to his side.
The first couple of scrums saw the visitors gain the upper hand and a penalty, but the Lions option to kick to the line for an attacking maul that ended up as a turnover to South Africa 'A'.
Tries were clearly going to be hard to come by with such ferocious defence and it was a charge down against the run of play that gained the home side an early advantage they never surrendered.
Eben Etzebeth charged down an attempted chip by Farrell with the Lions attacking on the SA 22, Damian de Allende regathered and sent Nkosi on his merry way.
This time the wing made no mistake with the corner flag and dotted down 75 meters from where the charge down occurred. Steyn kicked the conversion for a 10-0 lead 13 minutes in.
The SA 'A' side attacked strongly, but were held out twice before the Lions worked their way back up the field and into the opposing 22. They were rewarded with a Farrell penalty goal for their first points after 27 minutes, but also the knowledge that they could hold out the Bok drives.
They still needed to find a way how to keep Cheslin Kolbe in check though.
The wing gathered a misdirected kick near the halfway line and a step and a swerve later had Louis Rees-Zammit and Chris Harris grasping thin air. Kolbe released Lukhanyo Am for a beautiful try by the team captain. Steyn slotted the conversion and pushed the SA 'A' lead to 17-3 after 32 minutes.
Credit to the Lions though, who again surged up field with a number of drives and kept plugging away at the SA line. They opted for tapped penalties twice with South Africa under pressure and conceding penalties and from yet another sortie, De Klerk was deemed to have tackled without arms.
He was sin-binned and a minute later, was followed by Van Staden after yet another maul collapse by the green team and the flanker ruled as the culprit. South Africa held on, but the Lions went into the sheds knowing that they will be facing 13 men for the opening part of the second half.
The red jerseys started strongly and found the outnumbered SA side's defence stretching to breaking point. They worked their way upfield and it was only a matter of time before the Lions would cross the line. It was Wyn Jones (prop) who scored his first of the match and his team's first of the match, 42 minutes in.
That cut the lead to a converted try with another six minutes against 13 defenders.
Farrell kicked a long-range effort to cut the lead to four, but Van Staden's return at least meant that it will be fifteen aside for the last 30 minutes of the match.
Rees-Zammit went very close, but was deemed to have made a second movement as he stretched for the line and the SA side could escape from their red zone.
The SA 'A'-side had a good maul going and forced a penalty, went for another lineout, which earned them another penalty. Steyn stepped up but his 45m effort just missed.
The pendulum seemed to have slowly swung back towards South Africa, who tackled everything that moved in red with great passion and determination.
They decided to go for the lineout drive twice in short succession, with both attempts being repelled by the home unions side. An illegal tackle on Jasper Wiese gave his team another great lineout drive opportunity with eight minutes left, but again the only advantage the SA side got was the time being eaten by the stadium clock.
The Lions had a last chance in the final minute of play, but a handling error handed the ball back to South Africa, who cleared the ball from the resulting scrum to claim a hard-fought win.
South Africa ‘A’ 17 (17) – Tries: Sbu Nkosi, Lukhanyo Am. Conversions: Morne Steyn (2). Penalty goal: Steyn.
British & Irish Lions 13 (3) – Try: Wyn Jones. Conversion Owen Farrell. Penalty goals: Owen Farrell (2).