Casey is gaining a reputation for bouncing back from poor starts

His card was 11 shots better than his first round and 10 better than the average score at Oakmont this week.

The 29-year-old started his round in 104th place but ended the day in seventh, three strokes back.

"I've shot some very good rounds of golf but that has to be the best I think," the Cheltenham-born star said.

Interview: Paul Casey

606: DEBATE To put Casey's round in perspective look at the best rounds of the day JM

"This is possibly the toughest golf course I have ever played and I feel very, very lucky to have shot 66 on it.

"Sixty-six is way beyond my expectations, and I'm still a bit surprised at the score I shot."

To put his performance into some perspective, Casey's playing partners Trevor Immelman and Stewart Cink signed for a 79 and a 80 respectively.

The goal was to make the cut but I made a couple of birdies early on and the confidence sky-rocketed Paul Casey

And it was one of only two sub-par rounds in the entire 156-man field, along with Canadian Stephen Ames' 69.

Starting on the 10th, Casey birdied 10, 12, 14 and 17, before carding his only bogey at the hardest hole on the course, the 18th.

The world number 16 then parred the next six holes, birdied the 7th and ended with two more pars.

"It was a hell of a turnaround from a 77 yesterday when I really struggled with the swing," Casey added.

"It was really disappointing as I had put in a lot of preparation but I have a habit of starting poorly in big events.

"The goal was to go out and make the cut but I made a couple of birdies early on and the confidence sky-rocketed."