World No.1 Rory McIlroy admitted his first round at the Hong Kong Open on Thursday was a write-off and said he was suffering from fatigue, after he posted a three-over-par 73.

“I probably just lacked a bit of energy out there. I felt a bit lethargic – mentally, more than physically, if I am being honest,” he said, reflecting on the poor start to the defence of his Hong Kong crown.

In front of his watching girlfriend, tennis player Caroline Wozniacki, he had just the one birdie against four bogeys.

“I started off OK and just got more and more frustrated as the round went on. I just felt like I was hitting some decent shots and just not really converting any chances for birdies and I just let it get to me a little bit,” he said.

The 23-year-old from Northern Ireland has enjoyed a hugely successful year, his third place at the Singapore Open last weekend meaning he has topped the money charts in the United States and Europe.

“I suppose after last week, it was always going to be a bit of a let-down,” he added.

McIlroy said that he would put the round down to a bad day and return to Hong Kong Golf Club for the second round on Friday with renewed vigour.

“I feel like I’m playing well enough. I just need the head and the golf to match up and I’ll be OK.”

McIlroy said that the wind, which picked up in the afternoon, had also made life difficult.

“Hopefully I’ll go out there tomorrow morning and see if I can muster something a bit better,” he said.