England take on Mexico in Mexico City for a place in the World Cup quarter-finals on Monday morning as they aim to book their place in the Quarter Final.
In 89 competitive fixtures at the Azteca, Mexico have lost just twice and are unbeaten in 13 years. The stadium sits at around 2,240 meters (7,220 feet) above sea level which means players become more exhausted and struggle to keep their stamina up.
England will struggle immensely
It is a huge concern for manager Thomas Tuchel heading into the game as his players cannot adapt in time for the clash, giving them a huge disadvantage. Speaking to Snabbare, Mills believes that the Three Lions will seriously struggle to play at their best.
“In terms of quality of players, England have the better team. The Azteca obviously has an incredible atmosphere. What people do not understand is the effect of altitude. You can talk about it all you like, but unless you've been at altitude and trained there, you don't know what the experience is. It is incredibly difficult and takes a long time to acclimatise.
“People will say it's not too bad. I've been to Johannesburg, which is about 1,500 meters. Walking is not a problem at altitude, it doesn't affect the body greatly. However, the moment you start to do high-intensity work, the heart rate goes through the roof, your lungs start to burn, and you're gasping for breath.
“There's nothing you can do about it. You need to live and sleep at altitude for probably three weeks to get accustomed to it. Even then, it still has a slight effect and you cannot perform physically at your absolute best. You don't sleep particularly well at altitude either. You do not get that deep sleep you would at sea level.
“The Mexicans have been there probably since early June. Even if they've played games away from Mexico City, their training camp will have been at altitude. They are acclimatised and used to it; they can play at an intensity that other teams can't.
“I've been to training camps with my son and I'll go for a jog and think it's fine, but suddenly you up the tempo and it's like you get hit by a wall. It's very difficult and England will not be prepared for that.”
The last time they played a match over 7,000 feet was also their last trip to the Azteca in which Diego Maradona crushed England in the 1986 World Cup quarter-final.