Rory McIlroy and Shane Lowry shot matching 67s to sit three shots off the early lead on four-under-par at the PGA Tour's Cognizant Classic at PGA National in Florida.
Starting on the 10th, McIlroy got off to the perfect start with a birdie on the par-five, which he followed with six straight pars.
He finished that opening nine with a flourish however, knocking in a birdie two on the short 17th and then taking advantage of the par-five 18th to turn in 33 strokes at three-under-par.
"It was so benign," said McIlroy. "You're not going to get this course much easier."
Lowry also played the course the wrong way round and after opening with a bogey picked up birdies on 16 and 18 to turn two-under-par. The Offaly man would bogey his 10th hole but an eagle on the third and bogey on the fourth had him moving in the right direction.
Lowry dropped a shot on his penultimate hole but managed to sign off with a birdie to card his 67 and sit in a tie for 18th alongside his countryman and 11 other players.
Pádraig Harrington dropped three shots in his final half-dozen holes to card a frustrating opening round of even-par 71.
Having turned one-under-par, the Dubliner birdied 10 and 11 before giving both shots back with a double bogey on 12. A further dropped shot on the par-five 18th hole leaves Harrington level par heading into round two.
American Chad Ramey and South Korea's SH Kim shared the opening-day lead with seven-under-par 64s in Palm Beach Gardens.
Both Ramey and Kim went bogey-free, with Kim benefitting from an early eagle while Ramey rang up five of his seven birdies on the front nine.
Cameron Young, Andrew Novak, Ryan Moore, Austin Eckroat, Chesson Hadley and England's David Skinns were tied one stroke behind the joint-leaders.
Play was suspended due to darkness with six players left to complete the final hole of their rounds.
