VIRGINIA WATER, England (AP) -- Defending champion Rory McIlroy shot a 1-under 71 on Thursday, leaving him four shots behind clubhouse leader Robert Karlsson at the BMW PGA Championship after a mixed performance he blamed on mental fatigue.
Playing for the fourth straight week, the top-ranked McIlroy failed to recapture the form that saw him surge to a record seven-shot victory at the Wells Fargo Championship at Quail Hollow last week.
McIlroy had two birdies and two bogeys in his first five holes, missing a 12-foot putt for par at the first hole after failing to find the green with his approach. He went birdie-bogey-birdie from Nos. 12-14, and could not take advantage of par-5s on the final two holes.
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"Physically I feel fine but mentally I could feel myself getting down on myself out there, which is not something I have been doing the last three weeks," McIlroy said.
"I just need to be aware of that and keep everything on an even keel. It is acceptance of bad shots. Every time I have played well this year, I have been talking about how mentally good I have been. I feel like my patience was wearing thin out there today."
Karlsson made six birdies for a round of 67. The Swede birdied two of the final three holes to take the overall lead in the European Tour's flagship event.
"It was nice to put a good round together," Karlsson said. "I was careful off the tee. It is important round here to keep the ball in play. Hopefully this is the start of a good run."
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Karlsson was one shot ahead of four players -- Miguel Angel Jimenez, Jorge Campillo, Chris Wood and Y.E. Yang.
Stephen Gallacher of Scotland was forced to withdraw from the competition, shortly before he was scheduled to tee off with Lee Westwood and Tommy Fleetwood. Gallacher, who was part of Europe's 2014 Ryder Cup-winning team, has a wrist injury.