One word has dominated the build-up to England's World Cup last-16 clash against Mexico at the Azteca Stadium: altitude.
But there is an argument that the challenge facing Thomas Tuchel's side isn't even the greatest an England team will tackle this weekend.
The altitude - all 2,200m (7,220ft) of it - and atmosphere at the iconic Estadio Azteca arguably outweigh the challenge that awaits England on the field.
England must overcome tournament co-hosts Mexico, a side ranked six places below them in the FIFA World Rankings, to reach the quarter-finals.
No knockout match at a World Cup is easy, and when you combine all the factors - altitude, atmosphere and opposition - England's progress is far from certain, especially given their performances leading up to this stage.
But try contrasting that with what awaits England's rugby players several thousand feet above sea level in Johannesburg.
The standout fixture from the opening round of the inaugural Nations Championship sees Steve Borthwick's side travel to Ellis Park, the fortress of South African rugby, for an acid test against the reigning, back-to-back World Cup-winning Springboks.
Ellis Park, a venue where England's sole victory came in 1972, may sit at a lower elevation than the Estadio Azteca, a more modest 1,753m (5,751ft) above sea level, but it adds another dimension to what is currently the greatest challenge in world rugby.
"When you are at altitude there is reduced oxygen pressure in the air," sports physiologist Hannah Bashford told Your Site News.
"It means there is less oxygen with every breath you breathe and less delivery of that oxygen to your muscles. That makes it a lot harder.
"Physiologically, you see an increase in heart rate, breathing rate, and, mentally, with every movement on the pitch feeling a lot tougher for the England players."
To combat the effects of altitude, England arrived in Johannesburg 10 days before the match to acclimatise, a luxury not afforded to their footballing counterparts, who could only travel to Mexico City after Wednesday's round-of-32 clash against DR Congo and opted to fly in just over 48 hours before kick-off.
Modern technology means athletes have a series of measures at their disposal to prepare for the added physical exertion that awaits at altitude, such as cardiovascular activity while wearing an oxygen-restricted altitude generator.
England's rugby players have been put through their paces in simulations at around 3,500m, nearly twice as high as they will encounter in Johannesburg.
But as Bashford, based at Loughborough University, explains, it is those extra days spent acclimatising that could make all the difference.
"England could have used altitude chambers. You can use an altitude generator where you breathe through a mask, and it delivers the oxygen and you can replicate the conditions," she added.
"The key thing with altitude is the more you are exposed to it, the better you are going to be able to cope with it.
"The ideal is using simulated conditions beforehand and then getting out there as soon as possible, 10-14 days before, but that has not been possible [for England's footballers]."
England's footballers may have the altitude of Mexico City to contend with, but as England's record at Ellis Park over the last 54 years has shown, conquering the Springboks at altitude is a challenge they have yet to acclimatise to.
Starting XV: 15 George Furbank, 14 Immanuel Feyi-Waboso, Tommy Freeman, 12 Seb Atkinson, 11 Cadan Murley, 10 Fin Smith, 9 Jack van Poortvliet; 1 Ellis Genge, 2 Jamie George (c), 3 Joe Heyes, 4 Alex Coles, 5 George Martin, 6 Ollie Chessum, 7 Tom Curry, 8 Ben Earl.
Replacements: 16 Luke Cowan-Dickie, 17 Beno Obano, 18 Asher Opoku-Fordjour, 19 Charlie Ewels, 20 Guy Pepper, 21 Henry Pollock, 22 Alex Mitchell, 23 Marcus Smith.
