Matt Fitzpatrick off to flying start at Bay Hill as Rory McIlroy struggles

Matt Fitzpatrick produced a flawless round to earn a share of the lead as Rory McIlroy was left to rue a poor ball-striking day at the Arnold Palmer Invitational.

Fitzpatrick ended the opening day tied at the top of a high-quality leaderboard with Emiliano Grillo after both fired 67 in cool conditions at Bay Hill, but McIlroy struggled to a 74 while course-specialist Henrik Stenson was a shot worse.

Paul Casey strengthened the English presence in the top 10 as a flying finish lifted him to four under, and defending champion Jason Day returned from illness to card a solid 70 in early-morning temperatures he described as "brutally chilly".

Grillo recovered from bogeys at the third and fourth with four birdies over the next five holes, and the Argentine picked up another at the 11th before further gains at 16 and 17, where he holed a superb 40-foot putt, set a testing early target in the clubhouse.

He looked likely to retain the outright lead for the remainder of the day, but Fitzpatrick matched him in the late stages with a rare bogey-free round on a day in which only 27 of the 120-player field managed to break par.

Fitzpatrick started at the 10th and followed six solid pars with a birdie at 16, and he then knocked an excellent approach to 15 feet at the tough 18th and rolled in the putt for a three.

The Ryder Cup star holed from 25 feet for another gain at the first and converted a precise wedge to close range at the third before getting up and down from a greenside bunker at the sixth to pull alongside Grillo.

Fitzpatrick parred safely in to remain on five under, and he said: "I was just very consistent with everything. I actually didn't hit many close or have too many chances inside 15 feet, but I holed some good putts and that's important around here, so it was a good day.

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"I like that it's quite penal off the tee and missing the greens. If you really hit it well tee to green you can shoot some good scores. And I enjoy playing courses like this, it's a challenge."

Casey ruined a good start when he came up short with his approach to the eighth and found the water, costing him a double-bogey six, but he responded with a nice three at the 10th and raced back up the leaderboard with three birdies over the last four holes to match former US Open champion Lucas Glover's earlier 68.

Day enjoyed a steady opener as he offset three birdies with a lone bogey to sit just three off the pace along with last week's Valspar Championship winner Adam Hadwin, who was again playing nicely at four under until finding the hazard with a bad lay-up at the 18th and closing with a six.

Justin Rose was similarly consistent as he made two birdies, one bogey and 15 pars in his 71, but McIlroy's card was littered with errors after he reached the turn on one under.

The world No 3 bogeyed 10 and then pulled his second into water at the next as he ran up a double-bogey, although he almost holed his wedge approach to the 13th and tapped in for birdie.

A poor drive at 15 cost him another shot, which he got back at the long 16th, but he found trouble again at 18 when his second cannoned off the rocks just short of the green and into the rough, from where he took three more to get down.

Rickie Fowler also fired an error-ridden 74 which featured six birdies, two double-bogeys and four bogeys, while Stenson struggled to adapt to the bitterly cold early conditions and was one of many players to come to grief at the 18th, where he avoided the hazard but still managed to take six.