Former world number one Rory McIlroy had reasons to be fearful ahead of Friday's second round of the 78th US Masters at Augusta National.

McIlroy was satisfied with an opening 71 which left him just three shots off the lead held by American Bill Haas and two adrift of defending champion Adam Scott, Bubba Watson and Louis Oosthuizen.

But the 24-year-old was anticipating a tough course to get even tougher by the time he resumed his bid for a third major title in the final group of the day at 1359 local time.

"It was a good day at the office, anything in red figures was a good effort out there," said McIlroy, whose best finish in five previous Masters is a tie for 15th in 2011, when he led by four going into the final round but collapsed to a closing 80.

"When I saw the pin positions in the morning I thought they didn't want people to get too many under par.

"The set up was much more difficult than it has been in the past. Going out Friday afternoon the greens are going to be firmer. I wouldn't mind if they were a little softer. They're fast already. By Sunday they're going to be pretty dicey."

Watson carded the only bogey-free round of the day to finish three under alongside Oosthuizen, the man he beat in a play-off to win the green jacket in 2012.

But Scott's form was equally ominous, the Australian matching his opening 69 from 12 months ago despite a double-bogey five on the 12th, where he dumped his tee shot into Rae's Creek in front of the green.

The 33-year-old Australian is looking to join Jack Nicklaus, Nick Faldo and Tiger Woods in winning back-to-back titles, but said: "I haven't really (thought about it). It's been mentioned and it gets mentioned to every guy who is coming back to defend I'm sure.

"It's an amazing group of guys, great champions, but it's so much golf to play yet, I can't even think about it. My goal teeing off was to play three good days and get myself in contention and give myself a chance like I did last year."

Seven players shared fifth place on two under par, with another eight on one under including 54-year-old former champion Fred Couples, 50-year-old Spaniard Miguel Angel Jimenez, 20-year-old duo Matteo Manassero and Jordan Spieth and Scotland's Stephen Gallacher.

Gallacher held outright second place after a birdie on the ninth took him out in 33, and although he bogeyed the next three holes, a birdie on the 15th gave the 39-year-old an excellent 71 on his tournament debut.

"I'm a wee bit disappointed," said Gallacher, who was advised by putting coach Dave Stockton to treat it as the North Georgia Open. "It's a special place, one in a million place. It's just hard to get that out of the way and just concentrate on the golf.

"At the end of the day it's a golf tournament, so you've got to be trying to play the course."