Rory McIlroy promised never to make the same mistake again after a two-shot penalty cost him a share of the halfway lead at the Abu Dhabi HSBC Championship.

Northern Ireland’s US Open champion would have been alongside Dane Thorbjorn Olesen – a rather less well-known 22-year-old – but for brushing sand away from his line on the ninth hole of his second round.

That would have been all right if it had been on the green, but he was six feet off the putting surface and world number one Luke Donald instantly brought it to his attention.

“I wasn’t thinking clearly and just made a very stupid mental mistake,” McIlroy said following a level-par 72 that left him tied for fourth on five under par and part of a group which also included Tiger Woods.

Asked if he did not know the rule or just had a loss of concentration, he added: “I think it was a little bit of both – first week back as well. Luke said, ‘Don’t think you can do that’, and I was like, ‘Oh yeah, I can’t, can I?’. It happens and you just have to take it on the chin. There was so much sand in my line I didn’t even think about it – I’ll definitely not do it again.”

McIlroy certainly did not blame Donald for pointing it out and remembered what happened to Padraig Harrington in the same event last year.

The Dubliner shot an opening 65, but then was disqualified after an eagle-eyed television viewer spotted that his ball had moved a fraction on a green.

There was outcry about the unfairness of that and the rule was subsequently changed, but on this incident McIlroy said: “It’s a bit of a weird rule. You can move a loose impediment like a divot out of your line. You can’t move sand.

“Not a weird rule, but a tricky rule. That’s the same penalty as hitting the ball out of bounds. It’s tough, but the rules are the rules and we’ve got to play by them.”

While world number one Donald remained deep in the pack on one under with a 72, Woods is right in the thick of things heading into the weekend.