Defending champion Rory McIlroy— undisputed world No 1 and under no threat from a once debilitating back problem thanks to his intense physical fitness regime — is two shots behind leader Jordan Spieth after the opening round of the Emirates Australian Open in Sydney.

The Co Down man carded a two-under 69 at the Australian Golf Club as Spieth shot a four-under 67 to take a one-stroke lead over Australians Aron Price and Scott Gardiner with world No 2 Adam Scott seven shots off the pace after a three-over 74.

McIlroy, who edged out Scott with a 72nd hole to win the tournament last year, was level par for his opening nine holes where he birdied the 14th and bogeyed the 15th.

The 25-year-old then birdied the first and picked up further shots at the fifth and ninth, in-between his second bogey of the day at the seventh.

"It took me a little while to get going," said McIlroy, who also admitted he was feeling jet lagged. "I gave myself a lot of chances early on in the round and didn't really take them.

"The conditions were pretty tricky. It was tough to get the ball close to the pins with the wind and these greens being quite firm as well.

"I thought anything under par today was a decent score and it was nice to birdie the last and shoot something in the 60s. It puts me right there going into tomorrow.

"Golf tournaments are long, all you want to do is give yourself a chance going into Sunday. I definitely feel like there's a better score out there."

McIlroy explained in the build up to the event how last year's Australian Open win gave him the kick start he needed to set up a stellar 2014 season after a miserable 2013.

But he also went into detail about how his physical fitness regime has helped him become world No 1 and avoid a dengerative disc problem.

"It all started about three years ago when I was struggling with a bad back," McIlroy said in Sydney. "At one point in 2010 I was going for MRI scans every four weeks - just to see if I had a degenerative disc problem.

"While it wasn’t as bad as that, I have a lot of bone oedema around my L4 and L5 facet joints and the doctors said, 'If you don't take care of it, a stress fracture could occur and you'd be out for a year.'

"That's when it really hit me and I decided I had to do something about this, so I've been working out for about three years now. There was a lot of corrective work in the first six months, a lot of tedious stuff in the gym and that was quite boring."

McIlroy posted a picture of him doing squat thrusts with 275lb weights in Dubai, which prompted Justin Rose to post a funny "reply" with him straining to lift tiny weights at the end of a huge bar.

"But now I'm in a good place physically, I can move on and lift heavier weights, like the ones you saw last week," McIlroy added, explain that working out has now become part of his lifestyle.

"I've really started to enjoy it," he said. "It's become a bit of a lifestyle for me, being healthy, keeping active and staying fit. It's definitely helping my golf.

"If you look at the correlation between when I started doing this and my success on the golf course, you can see the results. I've become a lot stronger in my core, a lot more stable in my lower half. Flexibility and mobility is something I've never had to worry about but, obviously, I still stay on top of it."

Dr Steve McGregor, who has worked with Lee Westwood and other major sports teams in the past, is the man in charge of McIlroy's fitness regime.

"It's been a long process and I'm just thankful, touch wood, my body's in a good place now," McIlroy said. "Hopefully, that'll help me going forward and allow me play into my late forties, giving me a chance to prolong my career as long as I can."

As for life in a goldfish bowl, McIlroy he's utterly focussed on his golf and that the legal case with Horizon Sports Management is something that's at "the back of my mind, not the forefront of my mind."

"I have people who know what they are doing take care of that," he said. "I just dip in and out and plug in when I have to."

As for the huge demands on his time, he said: "My life is very structured to ensure I put adequate time and focus into my golf because that's what got me into this position.

"I say no to 95 per cent of the requests that come into us because there's not enough time. The stuff I feel I need to do or feel would be helpful to me, I do. I don't let anything take away from practice or preparation or trying to be a better player."

His focus is very much on winning the 2015 Masters and completing the career grand slam at the age of 25.

How often does he think of Augusta? 

"Every day," he said. "I think about it every day."