King suffers from an on-going knee problem which means he cannot train every day while his club Tottenham have to nurse him through from match to match.

The 29-year-old has moved up the pecking order following the injury to Rio Ferdinand which has ruled the England captain out of the tournament.

He could now line up alongside John Terry at the heart of England's defence in their World Cup opener against the United States on Saturday.

King said: "There is not a day goes by where you don't think about the knee. It's something I have to deal with.

"Everytime I get up and walk, there are restrictions with it. Squatting can be uncomfortable.

"I can't stretch my knee up to stretch my groin. I can't grab hold of my knee in that way. So there are definitely restrictions with it.

"I've had scans going back over the last two or three years and the knee has not really got any worse.

"It's nice to know that it's in a similar situation to when I had the operation, if not a bit better.

"But if it (carrying on playing) does impact on me in later life then it will have been worth it."

King added: "To be here is something I will never forget, to be involved in the World Cup 23 is something I will look back on.

"I missed out on the 2006 finals because I broke my metatarsal. You are a long time retired, so you might as well do everything you can now.

"That's the way I look at it. A career is short and I'm trying to get as much out of it as I can.

"No player really knows when their time is going to end, so I'm trying to concentrate on doing as much as I can."

King admits he savours every game he plays in because of the uncertainty surrounding his fitness.

It helps him to deal with not being able to train and having to spend the majority of time in the gym ensuring he is fit enough to be able to play.

He said: "You savour each game because you are aware how fragile your body is.

"That's why I don't do too much training because I see the games as the most important thing.

"I've got to try to find the balance between how much I can do and being available for a game.

"I'm trying to get as many games as I can and see where I go."

King is street-wise enough not to make any bold predictions about how his knee will hold up but pledges to give everything for the England cause in South Africa.

He said: "As a player, you don't know what's around the corner.

"Rio has had a lot of problems this season, but what happened was just an off the cuff incident with his knee.

"No player can guarantee their fitness. What I can guarantee is that I will give 100% and I'm ready to play if need be and I feel I can perform at the highest level."

King will be hoping to play some part in tomorrow afternoon's practice match with the Platinum Stars.

Defender Matthew Upson is making good progress after missing two days of training with a high temperature.