World number two Dustin Johnson shot a superb second round of 63 to take the halfway lead at the FedEx St Jude Classic on 10 under par while first-round leader Seamus Power followed up an opening 65 with a 69, including a double bogey on the par-4 ninth.

The Waterford man is currently four off the lead on six under par and leading the Irish challenge. Shane Lowry is one under and Padraig Harrington is on level par.

Johnson, the 2016 US Open champion made a slow start to his round after beginning on the back nine, wasting an opening birdie with dropped shots at the 12th and 13th holes, but righted the ship with a long birdie putt at the next and picked up another at the par-five 16th.

He holed out from 110 yards for eagle at the first and came within inches of repeating the feat at the next, settling for one of four birdies in a blistering second nine.

Winning the tournament would take Johnson back ahead of Justin Thomas as world number one going into the US Open and his efforts earned him a one-shot lead over fellow Americans Ryan Blaum and Andrew Putnam.

Blaum redeemed a bogey at the first with seven birdies in a 64 and Putnam matched that effort as both seek a first win on the PGA Tour.

Two-time Ryder Cup player Brandt Snedeker was three off the lead at seven under, one ahead of Chez Reavie with Brooks Koepka a further shot back.

Five-time major winner Phil Mickelson was four under after two birdies in his last three holes.