Masters champion Danny Willett produced a seven-under round of 65 to lead the way at the Irish Open, but has Rory McIlroy hot on his heels two shots back.
Willett, who has not played on the European Tour since his triumph at Augusta, got off to a solid start at The K Club, making gains at holes three and seven.
And a run of five birdies in six holes between the ninth and 14th - on which he produced a superb long putt - thrust him to the top of the leaderboard.
An uncharacteristic blip at the 17th, owing to a wayward tee shot, threatened to end Willett's round on a sour note, but the Englishman recovered to make amends with another birdie on the last after leaving himself with a simple putt.
"Things have been a bit mental but it's just great to be back in Europe playing golf, being around the lads that I've obviously played golf with for the last eight years," said Willett. "I'm very happy to be back here.
"We started off a little scrappy and then we just kept plodding along. Tried not to make too many mistakes. We had seen that there are a few scores in and around two or three under par, which we were looking at being a really good score.
"I just seemed to get hot with the putter on the back nine and rolled a few in, and not too many mistakes."
World number three McIlroy, who has missed the cut in this event in each of the last three years, remains in close pursuit after turning in a strong display in changeable County Kildare weather conditions, making just one bogey.
The Northern Irishman hit 17 of 18 greens on his opening round.
Callum Shinkwin and Jaco van Zyl are one shot down on McIlroy at four under, one ahead of Ross Fisher, Martin Kaymer, Brett Rumford and defending champion Soren Kjeldsen.