World No.1 and defending champion Rory McIlroy withdrew from the US PGA Honda Classic during his second round on Friday, saying he was struggling due to pain from a sore wisdom tooth.

McIlroy struggled through the back nine on Friday, his opening nine holes of the round, and hit his approach at the 18th into water. He then walked off the course and quickly departed the grounds with his coach and caddie.

“I sincerely apologise to The Honda Classic and PGA Tour for my sudden withdrawal,” McIlroy said. “I have been suffering with a sore wisdom tooth, which is due to come out in the near future.”

It was the first time in his career that McIlroy has withdrawn from a tournament and it comes as the 23-year-old from Northern Ireland struggles to find his form after switching to Nike equipment for this season.

McIlroy is a 6-1 co-favourite with 14-time major champion Tiger Woods in next month’s Masters, but the tooth issue could dim his bid to add to a major haul that includes the 2011 US Open and 2012 PGA Championship.

“It began bothering me again last night,” said McIlroy. “It was very painful again this morning, and I was simply unable to concentrate. It was really bothering me and had begun to affect my playing partners.”

McIlroy gave no hint as to how the injury might impact his plans for playing in the weeks leading up to the year’s first major tournament at Augusta National.

He had plans to play next week in a World Golf Championships event at nearby Doral and the Houston Open in the week before the Masters, but gave no hint if that was still the case in his statement, or in a later Twitter posting.

“Apologies to all at the Honda. A tough day made impossible by severe tooth pain,” McIlroy tweeted. “Was desperate to defend title but couldn’t play on. Gutted.”

Especially gutting for McIlroy was the fact the pain flared as he was trying to defend the title he won a year ago to put himself atop the world rankings for the first time in his career.