Rugby World Cup-winner John Smit admits South Africa are in a "dark hole" following their shock defeat to Japan on Saturday.

Karne Hesketh touched down with the final play of the match to seal a 34-32 win for the overwhelming underdogs in what has been described as the biggest upset the sport has seen.

And Smit - part of the Springboks team that lifted the Webb Ellis Cup in 2007 - believes Heyneke Meyer's men will need to win the remainder of their Pool B fixtures to earn a more favourable quarter-final draw.

"In terms of the permutations in the pool, as an optimist, if we do win the rest of our games in the pool we do follow an easier path to the final," he said in his role as Land Rover ambassador. 

"But it's a pretty dark hole to get out of, so they've got a lot to do in a couple of days.

"The one thing is the South African team will have to do a lot of soul searching but they will get better after this and they certainly know that it is do or die.

"They're on the back foot now but they'll have to fight their way out of it."

South Africa next face Samoa in Birmingham on Saturday.