From Brian Keogh in Vilamoura

Frustrated Rory McIlroy admitted he was mentally “frazzled” as he was left making up the numbers in the Portugal Masters at Vilamoura

The Holywood ace, 18, opened with one under par 71 at the Victoria Club but found himself a massive 10 shots off the scorching pace set by Germany’s Martin Kaymer.

The Dusseldorf rookie, 22, carded a blistering, 11 under par 61 in perfect morning conditions to equal the lowest round on tour this year and take a three shot lead over Argentina’s Daniel Vancsik and Swede Martin Erlandsson.

And McIlroy confessed that he got frustrated for the first time as a professional due to mental tiredness.

He said: “It’s my sixth week out of seven so I am a little bit frazzled. I let a few things get to me out there and that is just tiredness.

“Overall it wasn’t very good but I’ll try to improve on it tomorrow morning, make the cut and hopefully have a good weekend.

“It’s okay - similar to the first rounds I had in the Dunhill Links and Madrid. I just didn’t play very well and struggled with my ball-striking and putting.”

Peter Lawrie leads the Irish challenge on five under par after a six-birdie 67 and warned that Kaymer will find it tough to follow his opening effort in afternoon conditions.

Lawrie said: “I don’t think I’ve ever shot five under and finished up six behind the leader in the first round. That hasn’t happen me since my pitch and putt days.

“Let’s see how the leader does in the afternoon when the greens are as firm and crusty as they were today.

“I’ll try to put in a decent score in the morning and see where that leaves me. In the afternoon the greens are firm and it is very hard to stop the ball close to the pins.”

It was a similar story for Kilkenny’s Gary Murphy, who followed Lawrie into the scorers hut to sign for a four under par 68 while Paul McGinley and Damien McGrane shot 69.

McGinley was happy with his effort until he saw Kaymer’s score on the leaderboard as he finished.

Shaking his head, McGinley said: “You shoot 69 and come in a see a 61. It's a golf course that is over 7,000 yards long and the people at home will be looking at the coverage and thinking it must be a pitch-and-putt course.

"But there are a lot tough shots out there and a bit of grain in the greens. It just shows you the quality of golf we have on the European Tour.”

Darren Clarke showed signs of a return to form with a two under par 70 but Order of Merit candidate Justin Rose was left eight shots adrift after a roller coaster 69.

Needing a top two finish to leapfrog Padraig Harrington and Ernie Els at the top of the Order of Merit, Rose followed birdies at his first two holes with a double bogey seven at the 12th before recovering with three three back nine birdies.