Rory McIlroy hit four birdies in a five hole spell to move into contention for his first US victory at the Honda Classic in Florida.

Starting on the back nine, the Ulster teenager dropped shots at the 12th and 15th but birdied the 18th, first, third and fourth and then parred his way home to card a 68 that left him just five shots behind surprise halfway leader YE Yang of Korea on two-under par.

"I got off to a slow start but the back nine is pretty though out here," McIlroy said. "I played okay, though I didn't really feel comfortable today. Nothing was happening and then I made a few birdies. I sort of turned it around and 68 was a good score out there."

McIlroy opened his first strokeplay event in the PGA Tour with a level par 70 that could easily have been several shots better had he not run up a triple bogey six at the par-three 17th.

But for a while he looked destined to make up the numbers before he moved through the gears around the turn to remain very much in the hunt for victory.

One of the early starters at the tough Jack Nicklaus design, the 19-year-old Ulsterman three-putted the 12th for bogey, missed holeable birdie chances at the 13th and 14th and then failed to get up and down for his par from greenside sand at the tough, par-three 15th.

But he steadied the ship with an excellent sand save at the 16th and then got up and down from another bunker at the par-five 18th, this time for birdie, to get back to one-over par for the tournament.

Turning for home, the world number 16 holed an excellent six-footer for a birdie at the first, saved par with yet another up and down from sand at the second and chipped to just two and half feet to set up an easy birdie at the par-five third.

At the 376-yard fourth he blasted a 319-yard drive and and knocked his pitch to three feet to get to two-under par before parring his way home for a 68 that left him tied 10th place at that stage - just four strokes behind overnight leader Robert Allenby (68) of Australia and American Will MacKenzie (67).

But he eventually finished the day in a share of 13th as world number 460 Yang posted a second-round 65 to lead by a stroke from Allenby, Mackenzie and Jeff Overton.

World number two Sergio Garcia and Sweden's Fredrik Jacobsen were in a group tied at one under with Darren Clarke firing a one-over par 71 that featured a nightmare triple bogey six at the 17th.

The final hole of the three-hole stretch known as the Bear Trap has not been a happy hunting ground for either Clarke or McIlroy over the first two days.

Just 24 hours after McIlroy failed to escape from the edge of the lake and then put his third over the green into a bunker before taking three more to hole out, Clarke did it in reverse.

In the left, greenside trap off the tee on the 190-yard tester, the five-time Ryder Cup hero knocked his second through the green into the lake and then failed to hole a 12 footer for a double bogey after being forced to take a penalty drop.

Bidding to haul himself into the world's top 50 over the next few weeks so he can qualify for the Masters, Clarke made the three-over par cut with two shots to spare.

But it was a scramble for defending champion Ernie Els of South Africa, who was four over playing the par-five 18th but holed a seven footer for a birdie to progress.

However, Camilo Villegas and Justin Rose crashed out after rounds of 74 and 79 left them on seven-over and 12-over par respectively.