Too much to do: Rory McIlroy scores well but it probably not enough

The highlight of the 40-year-old's round was an eagle after driving the 273-yard par-four 15th at TPC River Highlands in Cromwell, Connecticut, but a further six birdies took him to 16 under, well clear of second-placed American Russell Henley.

"That's something I rarely see from me, 18 greens in regulation, so whatever he (swing coach Peter Kostis) told me worked," said Casey in his post-round interview.

Casey, who won the Valspar Championship in March, has world number nine Jason Day and two-time Masters champion Bubba Watson six shots behind but his potential Ryder Cup team-mate Rory McIlroy appears too far back to challenge.

A second successive 69 for the Northern Irishman was well off the pace, with most of his rivals scoring in the mid to low 60s, and at eight under the four-time major champion has probably left himself too much to do.

He too eagled the 15th, chipping in from the front of the green, but a bogey at the last - his third of the day after two earlier birdies - was a disappointing way to end.