Captain David Beckham agreed his side had faded as England beat Paraguay 1-0 in their opening group game on Saturday.
He said: "You don't realise how hot it was out there.
"We were good at times but we gave away the ball too much. But it was hot and we got what we wanted.
"The performance could be better but we will work on that. We set out to get the three points and we did that.
"We let them pass the ball around too much, but we were exhausted.
"We can put it down to that because we are usually strong and our fitness levels are high."
