A flood of "nasty" e-mails has made Ryder Cup star Paul Casey realise he made the biggest mistake of his career so far by talking about hating Americans.

The fall-out from Casey's comments to a Sunday newspaper caused him to lose sleep on the eve of the World Cup in Spain - although it did not prevent him and partner Luke Donald scoring an 11-under-par 61 in the opening fourballs.

"I've had lots of comments," he said. "Most of them I won't repeat, but 99.9 per cent of them have made me regret what I said.

"The story's run with, though, and there's nothing I can do to correct it. If I could I would.

"I have nothing against the United States whatsoever and I wish I could take it back. I struggled to get to sleep and it weighed on my mind this morning.

"It's made people angry, which I fully understand, and it's made me angry and upset. I've had a huge negative feedback and nasty stuff on e-mail.

"The word 'hate' was my error. I shouldn't have used it."