In desperation, sheer gut-wrenching desperation, South Africa had to use their entire big-name bench to save the day against Tonga and stay out in front of Pool A. The warning to England, the Pacific islanders’ next opponents on Friday, is undeniably clear.

If South Africa’s destiny is to snatch the World Cup ahead of longer-term favourites New Zealand, it will have hung heavily on events in a second half that saw Tonga open up a 10-7 lead just four minutes after the teams turned around thanks to a try from Perpignan tighthead prop Kisi Pulu.

As the Springboks absorbed the score, in the gloom under the stand, six of their bench, all old hands at the winning game, stood grim faced, readying themselves for action. On they went, signifying that the rescue mission was underway. So effective was the early stages of their arrival that in the space of 11 minutes South Africa had scored three tries and a penalty and were heading for a comfortable victory at 27-10.

But if John Smit, Victor Matfield and Bryan Habana thought their arrival would lead to long-term security they were wrong, remarkably wrong. It required nerves and control in the final stages for the two teams to hang on to what they had, and South Africa could not have argued if Tonga had squeezed the result away from them as the underdogs stretched for the corner in the final move only to see the ball bounce right and out of danger instead of left into a tense last few seconds.

South Africa had decided that their plan was to give their senior players a break, realising that if they did not do so they might feel the effects of perpetual rugby in the final stages of a tournament.

The new team did not match up in the first half and instead of constructing a reasonable lead were 3-0 behind to a penalty by Pierre Hola after nine minutes, and produced their only score after 18 minutes when full-back Ruan Pienaar tapped on from 40m out to beat the defence. That was converted by Andre Pretorius – but his goal-kicking was otherwise poor as he missed four penalty attempts.

Tonga must have realised at half-time that they had avoided the possibility of South Africa running away with the game. There were times when Nili Latu, Tonga’s captain, and his pack were right in their opponents’ faces and the sense of bounce and optimism among the Tongan backs was considerable.

The fact that Tonga did not score a try in that first half was wiped out at the start of the second when Latu led a significant thrust for the line. When it was checked Tonga reformed, drove again with increasing intensity, and Pulu surged for the try-line. Hola’s conversion securing the 10-7 lead.

The Springbok replacements – five of them arriving after 48 minutes with Percy Montgomery coming on after an hour – must have been delighted with their work as a penalty by Francois Steyn and tries from Juan Smith, Bobby Skinstad, and a second by Pienaar left the Springboks leading by 17 points inside the final quarter.

Steyn and Joseph Vaka were then yellow-carded for fighting and Habana, and all his menace, was sin-binned after 69 minutes for holding on after a tackle. Tonga gained a second try through Suka Hufanga in the corner one minute after Habana’s yellow card and added another through Viliami Vaki two minutes later – from a move that started when Montgomery missed the chance to kick ahead.

But Montgomery did kick a precious penalty after 76 minutes to preserve the margin at eight points and Tonga could only reply in kind through Hola with two minutes left.

“We saw South Africa make a lot of changes and we decided to put out our best team and were confident that if we stuck together we could do well,” said Latu. “England are not world champions for no reason. It is going to be a big night for us [on Friday], it’s do or die next week.”

Jake White, the South African coach, said: “I’m relieved, all credit to Tonga, they were outstanding, especially the way they came back at us after we had such a big lead. We always knew that the third week in this tournament was going to be a tough one and we always planned to give some guys a rest in week three.”

Star man: Bobby Skinstad(South Africa)

South Africa:R Pienaar (Sharks); A Willemse (Lions), B Habana (Bulls) 48min), W Olivier (Bulls), W Julies (Bulls, F Steyn (Sharks) 48min), JP Pietersen (Sharks); A Pretorius (Lions, P Montgomery (Sharks) 60min), R Januarie (Lions); G Steenkamp (Free State, BJ Botha (Sharks) 48min), G Botha (Bulls, J Smit (Sharks) 48min), CJ van der Linde (Free State), B Botha (Bulls), A van den Berg (Sharks) V Matfield (Bulls) 48min), W van Heerden (Bulls), D Rossouw (Bulls, J Smith (Free State) 25-31, 50min), B Skinstad (capt, Sharks)

Tonga:V Lilo (Fanga’o Pilolevu); T Tu’ifua (Counties Manukau), S Hufanga (Brive), E Taione (Sanyo Wild Knights, I Tupou (Kolofo’ou) 77min), J Vaka (World Fighting Bull); P Hola (Kobe Steel), S Tu’ipulotu (Yokogawa, S Havea (Marist) 67min); S Tonga’uiha (Northampton), A Lutui (Worcester, E Taukafa (Lyon) 67min), K Pulu (Perpignan), P Hehea (Metro Racing, Kauhenga 67min), E Kauhenga (Ricoh, I Afeaki (Grenoble) 50min), V Vaki (Perpignan), N Latu (capt, Bay of Plenty, L Filipene (IBM Big Blue) 66min), F Maka (Toulouse)

Yellow cards: South Africa:Steyn (63min), Habana (69min)

Tonga:Vaka (63min) Tries: Pienaar 18, 65, Smith 59, Skinstad 63 Conversions: Pretorius, Montgomery Penalties: Steyn, Montgomery

Try: Pulu 44, Hufanga 70, Vaki 72 Cons: Hola (2). Penalties: Hola (2)

Referee: W Barnes (England). Attendance: 40,069