Rory McIlroy got a brief taste of the kind of rollercoaster back nine he might experience at the Masters when he caressed the lead only to wobble down the stretch and end up with his title chances hanging by a thread — all in the space of an hour.

There's still a round to go in the Arnold Palmer Invitational at Bay Hill but as he watched Henrik Stenson finish with five successive threes — the Orlando-based Swede played the last four holes in four under par for a 66 and a two shot lead over Morgan Hoffman on 16 under — McIlroy admitted that he may need a final round miracle to clinch his second win of 2015.

There were no fireworks on the front nine, where he made five birdies in a row on Friday having started on the back nine. 

In fact, his first birdie didn't come until the par-five sixth but it was a tap-in after he'd missed an 11 footer for eagle.

While he chipped in from the back of the ninth green for his second birdie of the day and then looked to be building up a head of steam with birdies at the 12th and 13th gettting within a shot of the lead on 12 under par, he never looked 100 percent happy.

The 25-year-old bogeyed the par-three 14th after coming up short of the green with his tee shot and dropped another shot on the next, three-putting from 30 feet after an excellent escape from the trees.

He then duffed a chip from right of the par-five 16th green having driven into sand on the right and compounded the error with another three-putt.

"I'm frustrated," McIlroy admitted to Sky Sports 4. "I birdied 12 and 13 and got right into the thick of things and then to give three shots back in quick succession was not really the plan. 

"I'm disappointed with the way I finished but I played well for the most part. I need to try to get off to a good start tomorrow, shoot a good score and see where that leaves me.

"There's a lot of quality players above me. I see Henrik just eagled 16.... So I would need to get off to a very fast start (to win) — 30 or 31 on the front nine it's possible; just try to start better and go from there." 

McIlroy is seven behind Stenson and five adrift of Hoffman with a round to play in his final warm up event before the Masters.

Stenson carded his second consecutive 66 and played the last four holes in four under par for the second day running as well, following a birdie on the 15th with an eagle on the 16th and another birdie on the last.

"It was nice," the Swede told Sky after finishing 16 under par, two ahead of overnight leader Morgan Hoffmann with defending champion Matt Every, Jason Kokrak, Matt Jones and Ben Martin another shot back.

"I managed to sneak a couple of birdies on the front nine and then had a couple of unlucky breaks in the middle, but stayed patient again and got a really nice finish again. Four under for the last four, same as yesterday - I don't mind keep doing that round here." ​

Pádraig Harrington bogeyed the third and then missed a couple of five footers for birdie to turn in one over before coming back in a rollercoaster two under 34 for a 71 that saw him slip to tied 29th on six under.

Two over for the day after a three-putt bogey at the 11th, he then birdied four of the next five before dropping another shot at the 18th.

After holing a bunker shot at the 12th, he chipped in for birdie at the 14th, holed a 30 footer at the tough 15th and then two=putted the par-five 16th for a hat-trick of birdies before being forced to lay up from the right rough at the last.

Harrington's birdie blitz was impressive but the real fireworks came from the man he beat in that play-off at the Honda Classic — Daniel Berger.

The American recorded the first albatross in tournament history on the sixth. 

"I had 240 to the hole, a little right to left wind, just grabbed a four iron and hit an unbelievable shot," Berger told Sky Sports 4. 

"I didn't seem so excited after it went in because I wasn't sure if it went in or not but it was pretty special. It was cool but I had to calm down and get ready for the next hole." 

Berger also holed from 40 feet for an eagle on the par-five 16th and recorded three birdies and four bogeys in an eventful 68 to finish seven under.