Seamus Power leads at the St Jude Classic in Tennesse after opening with a blistering 65.
The Waterford man who went to university in Tennessee, landed six birdies and just a single bogey in a round of five-under.
Power was 11 shots clear of a group of finishers that included major winners Phil Mickelson and Brooks Koepka.
Padraig Harrington also hit six birdies but a pair of bogeys on 17 and 18 brought him back to two under overall. Shane Lowry had a two-over 72.
A week before trying to complete the career grand slam at the US Open, Mickelson became ragged off the tee late in his round but avoided any damage at TPC Southwind in Memphis.
"I finished the round off, even though I didn't quite have it there at the end," Mickelson told PGATour radio. "I had a nice simple round going and then I missed the last four fairways.
"I was able to gather myself well and control my thoughts a little bit better."
Mickelson, 47, said after a first round 74 at last week's Memorial that his focus had been lacking, but he subsequently gathered himself to tie for 13th.
By playing in Memphis, Mickelson is continuing his traditional build-up for the U.S. Open, before which he usually likes to play two weeks straight.
A six-times US Open runner-up, he has had three chances since winning the 2013 British Open to become the sixth player to complete a career grand slam, but he has not come close.
He even skipped last year's event - won by Koepka at Erin Hills - to attend his daughter's high school graduation.
Koepka matched Mickelson's 66 on Thursday, saved by a putter that was almost as hot as the Memphis weather.
2016 US Open Champion Dustin Johnson is among the pack on three-under.
