Veteran lock Simon Shaw has plenty to smile about after being promoted to the Lions' Test line-up © Getty Images

The sun will also be welcomed by the Lions when they eventually return to the Highveld but with that joy will be tempered by the prospect of playing the most important game of their lives at 1400m above sea-level.

Put simply, playing at altitude makes breathing harder as the thin air means there is less oxygen available.

The issue has troubled most touring teams in South Africa and the Lions are no different. Lions fly-half Ronan O'Gara memorably summed up the debilitating effects of playing at altitude following the opening clash with the Royal XV in Rustenburg.

"I felt like an imbecile in that first match," he said. "I certainly underestimated the effects of altitude. The mind was telling me one thing but the body wouldn't get me into the position to do it."

As with every other aspect of this tour, the Lions management and medical teams have put months of preparation into this part of the tour. And reports suggest that England football coach Fabio Capello has already sought their advice ahead of next year's Fifa World Cup.

The Lions' preparations were to include a training camp in Spain where conditions would be similar to those they would face here in South Africa but that was subsequently scrapped. Instead the players were encouraged to use altitude simulators - adding a sci-fi element to their pre-tour build-up at their leafy Surrey base.

"The Lions will discover the difference when their chests start burning," warned Springboks skipper John Smit earlier this week but perhaps he as unaware that the Lions have continued their altitude training whilst at sea-level thanks to the high-tech equipment available to the fitness team. One such training method requires players to wear a mask connected to a machine that limits oxygen intake.

The Lions have played their last five games at sea-level in Cape Town, Port Elizabeth and Durban and have chosen to prepare for the must-win second Test in Cape Town before flying up to Johannesburg on Friday - the day before the game.

Experts say that ideally you need eight days preparation at altitude to combat the effects but the Lions' schedule did not allow for that. The alternative is to travel up at the last possible moment - as the Lions are doing. The science behind this approach says that the somewhat crippling effects do not then have a chance to kick in.

Only time will tell if the Lions have got their preparation just right. But of course, the altitude issue is just one factor - albeit a major one.

The teams for the Pretoria showdown have both been announced with South Africa opting for only one change. Schalk Burger returns to the Springboks' line-up for the unlucky Heinrich Brussow who drops to the bench. However the Lions, who must win the game to keep their feint hopes of a series victory alive, have opted for a little more surgery with five changes including a first Test cap for veteran lock Simon Shaw.

South Africa have only lost three of their last ten clashes at Loftus Versfeld - all to New Zealand - and England were the last northern hemisphere side to upset the odds back in 1994.

The odds are really against the Lions and the first sell-out of the tour suggests that the home fans are confident of seeing their side complete the job.