Wales flew out of Wellington on Monday ready to regroup and prepare for next Sunday's Pool D showdown with Samoa in Hamilton.
If the South Sea Islanders triumph - as they did against Wales at the 1991 and 1999 World Cups - then Charteris and company will be left hovering on the brink of making a pool stage exit.
It is a sobering possibility for a team that produced an immense performance in outplaying South Africa for large parts of a punishing contest.
But if they can maintain the form that almost accounted for the Springboks, then the minimum World Cup target of a quarter-final place should be attained.
"It's the World Cup, and unfortunately we are playing knockout rugby now," said Wales lock Charteris.
"We can't lose again. We have got to win all our games through to the final.
"It's heads up now for Samoa. It is going to be another brutal game straight off, and we have got no choice but to focus on that one now.
"We knew what a tough pool we are in - there is no easy game in this group.
"We were hoping to get a win (against South Africa) and get a cushion, but that didn't happen, so we have got to go to plan B.
"We need to ensure we get second spot, or keep the pressure on South Africa, and hopefully one of the other teams will turn them over and open up the pool again.
"It's easily possible. If they are all going to be tough games for us, they are all going to be tough games for the Boks as well."
Twenty four hours on from the numbing 17-16 loss, and Wales had started the process of picking themselves up, buoyed by the possible availability after injury of fly-half Stephen Jones and prop Gethin Jenkins.
And they could also reflect on a succession of eye-catching displays by their younger players, notably the back-row trio Dan Lydiate, skipper Sam Warburton and try-scorer Toby Faletau.
"Toby was immense," said Charteris, of his Newport Gwent Dragons and Wales team-mate.
"That was probably the best game I have ever seen him play, for Wales, the Dragons, for anyone.
"As a young kid on that stage, he took it all in his stride. I cannot speak highly enough of him - it was an unbelievable performance.
"It is pretty hard to sum it up, really. We didn't start the game well, gifting them seven points in the first 10 minutes. That is what killed us."
World Cup organisers, meanwhile, say "correct protocol" was followed by the match officials during Sunday's clash over a disputed James Hook penalty kick.
Wales full-back Hook's 14th-minute strike appeared to go over successfully, but assistant referees Vinny Munro and George Clancy kept their flags down.
Hook said later he thought the kick was successful, but the television match official Matt Goddard was not consulted by English referee Wayne Barnes and Wales went on to lose a thriller by one point.
The Wales camp, led by coach Warren Gatland, did not make an issue of an incident which ultimately cost them dear, and they retained that stance on arrival at their hotel overlooking Lake Taupo.
In a statement, tournament organisers said: "Rugby World Cup Limited has clarified the status of the television match official protocol following Sunday's Pool D match between South Africa and Wales.
"Under protocol, the referee may consult the assistant referees or the television match official if he is unsure as to whether a penalty kick, drop-goal or conversion has been successful.
"During the match in question, the match official team felt at the time that there was no need to consult the TMO following a Wales penalty kick, as they were confident that the kick was not successful. Correct protocol was therefore followed.
"The IRB (International Rugby Board) will not be making further comment."
Barnes is due to referee Wales' final Pool D game against Fiji in Hamilton on October 2.
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