It was also three better than favourite Ernie Els and four better than Tiger Woods, although Woods still had six holes of his first round to play.

According to Mickelson, Casey could have been even further ahead of him.

“That was an easy five under,” said the American.

“The way he played I wouldn’t be surprised one bit if he is on top on Sunday.”

Casey, whose score was matched by France’s Thomas Levet just four days after the Scottish Open victory which gave him a place at Troon, was thrilled at erasing the memory of that nightmare opening at Sandwich last year.

“It’s a sign I’m more relaxed and not getting in my own way now,” said Casey.

“Major championships are a big deal, but you have to treat them as every other event. If you don’t then 85s happen.

“If you try too hard you don’t succeed. You just have to try the right amount. You’ve got to want it a lot, but not push it over the edge and find a happy medium.

“We found it a week in April and hopefully we’ll find it for a whole week here.”

That was a reference to his sixth place on his debut in the Masters at Augusta, by far the best performance of his career in the majors so far. He has played only six others – and missed the cut in five of them.

Casey is aiming big, though. He openly admits that his target is to join the elite group of Gene Sarazen, Ben Hogan, Jack Nicklaus, Gary Player and Tiger Woods who have won career Grand Slams.

That, he believes, set him apart from some of his colleagues on the European Tour.

On Tuesday, Casey accused them of showing “the George Best syndrome” and, despite some backlash, he was not backing down from his comments when questioned again.

“Maybe a few guys are too content being journeymen,” he stated. “I want to play on the world stage, play against the best. Why would you want to be content with just playing on the European Tour?

“There are definitely a few who have potential and don’t want to capitalise on it.”

Casey, who won all his four games in the Walker Cup victory over America at Nairn in 1999, turned professional after finishing second in the world amateur team championship and was a college star at Arizona State.

That was where he first met Mickelson and since he has kept a home in Scottsdale they have played many rounds together.

Casey said: “I’ve only beaten him once or twice. I still have a lot of catching up to do, but when I was paired with him I thought it was fantastic. And Shigeki (Maruyama) is a lot of fun too.

“The goal tomorrow is try to out-smile him.”

As well, of course, to repeat a round in which he made up for bogeys on the ninth and 12th with birdies at the first, fourth, eighth, 10th, 11th, 16th and 18th. The final two came with putts of 30 and 16 feet.

A smile has not left Levet’s face since he shot his closing 63 at Loch Lomond and suddenly had to change plans for a week off.

The 35-year-old, beaten by Ernie Els at Muirfield two years ago only at the fifth hole of a play-off, had four birdies in five holes from the fourth and had the added bonus of a 33-foot putt on the 222-yard 17th.

“I just hope my game stays the same and my mind stays the same,” he said. “When you play that well you don’t really feel the pressure because you know you’re going to make things happen.

“I must have heard ‘well done at Loch Lomond’ about 50,000 times this week already. It’s nice.”

Maybe it will be third time lucky for the French following his own near miss and Jean Van de Velde’s self-destruction at Carnoustie in 1999 from three ahead with one to play.