Rory McIlroy lost his swing and his putting touch to finish the opening day seven shots off the lead in the Arnold Palmer Invitational at Bay Hill.

Just 27 players broke par at the demanding Orlando venue, but they didn't include the world No. 3, who carded four birdies, four bogeys and a double bogey six in a two-over par 74 as Graeme McDowell shot a 72.

Seven shots adrift of Emiliano Grillo and Matthew Fitzpatrick, who carded 67s, McIlroy headed straight for the range to work on his swing as he ended the day tied for 58th.

"Very disappointing," McIlroy said after a closing bogey at the 18th, where he was fortunate to ricochet off rocks into a greenside bunker rather than the lake on the right.

"I didn't hit the ball the way I wanted; I lost a few shots to the right and put myself out of position quite a few times." 

McIlroy wasn't helped by an ice cold putter either

The longest putt he holed was from just inside five feet at the 11th, and that was for the double-bogey six that came when his ambitious, 200-yard fairway bunker shot came up short in the lake left of the green.

That mistake was a hangover from the 10th, where he three-putted from the fringe from 18 feet.

"I was playing okay; I was managing it. And then I felt like I hit two good shots into 10 and walked off with a bogey there," McIlroy said. 

"And sort of a bit of a mental mistake at the next, I let that linger a bit and made double. 

"The course is playing pretty tough this year, but I feel like today I, even though I didn't play my best, I should have shot something under par, I guess. 

"But I'm going to go work on a couple of things on the range tonight and hopefully straighten it out for tomorrow."

McIlroy started his day on the back foot, three-putting the par-three second from 43 feet before moving into the red with chip and putt birdie fours at the fourth and sixth.

When he got up and down for par from 84 yards at the eighth after a bad tee shot, a good score seemed likely.

But he was never comfortable with his swing on a testing set up and his lack of feel on the greens turned what might have been a level par round into a 74.

"My upper body's outracing the club, and I'm leaving the club face open, and I just need to square it up," McIlroy confessed. 

"I hit a few better shots on the way in, so just try to get a few more of those feelings on the range and try and hit it better tomorrow."

"I didn't putt very well today, I didn't hole much, but my chipping was pretty good, bunker play, that stuff was decent. But I just need to put myself in position a little bit more off the tee, and if I can do that, then I've got chances to score."

After going bogey - double bogey at the 10th and 11th, he almost holed his 146-yard approach to the 13th and tapped in from 18 inches to get back to one-over par.

A poor bogey at the 15th set him back again, and after missing an nine footer for eagle at the 16th, his approach to the 18th was killed by a switch in the wind, and he ricocheted off the rocks short of the green into a bunker left of the green.

From there he splashed out to nine feet, but he never threatened the hole with his par putt.

McDowell had three birdies and three bogeys in a 72, single-putting ten times to go into the second round tied for 28th.

As for Grillo and Fitzpatrick, their five under par 67s gave them a one-stroke lead over Lucas Glover, Paul Casey and Charley Hoffman.