(CNN) -- Rory McIlroy was at a loss to explain another first round meltdown after the World No.2 made a sorry start to the Memorial Thursday.
McIlroy carded an opening six-over 78 at Muirfield Village -- littered with errant tee shots and missed putts -- to leave him 13 shots behind leader Charl Schwartzel of South Africa.
The 24-year-old Northern Irishman came to Ohio off the back of a missed cut in the European Tour's flagship event at Wentworth and after a first hole birdie when starting on the 10th, his day deteriorated quickly.
He four-putted the 12th hole from 58 feet for double bogey and added three more bogeys to go to the turn in 40.
Tiger Woods' former coach Butch Harmon tweeted: "What is going on with Rory, already +4 thru 9 holes at the Memorial ??????? WOW !!!!!!!!"
Two more shots were dropped on his second nine, concluded by a dispiriting miss from four feet for a birdie at the last, taking his tally of putts for the round to 33.
"The last four weeks have been the same," McIlroy told the official PGA Tour website.
"I've missed a lot of short putts. It's probably lack of confidence more than anything else. And those are the sort of putts that are important to keep the momentum of the round going. And they're the putts that I'm not really making."
But despite showing some improvement on the U.S. Tour with three top 10s in his last four starts, McIlroy admitted he was still out of sorts.
"The game just isn't all there at the minute. But I'm working hard and I'm trying to figure things out and hopefully they'll come around soon."
He was trying to stay positive in what is likely to be his last tournament before the U.S. Open at Merion. "A few bad rounds of golf isn't going to ruin anything," before admitting -- "but I don't really have many explanations for this."
While McIlroy searches for his best form, former Masters champion Schwartzel shrugged off a late double bogey on his way to a seven-under round which included 10 birdies.
"The birdie on the last definitely will make my evening a lot better," he said.
He led Scott Piercy by a shot on a day of generally low-scoring at the Jack Nicklaus-hosted tournament.
Defending champion Tiger Woods was one of the later starters, bidding for his sixth victory in one of his favorite events, but found it difficult to gain momentum.
A late birdie on the 17th took him under par as he closed with a 71.