(CNN)He might be World No. 1, but Rory McIlroy is starting to make a habit of throwing golf clubs.
McIlroy admitted he was "feeling myself getting a bit angry out there" after hurling his club down a fairway during the opening round of the BMW PGA Championship at Virginia Water on Thursday.
In March, McIlroy also threw his 3-iron into the water on the eighth hole at the Cadillac Championship at Trump National Doral in Miami.
The defending champion let his frustration show as he hit a one-under 71 Thursday, leaving him six shots behind early leader Francesco Molinari.
Irishman McIlroy -- in action for a fourth successive week -- fell far short of the inspired golf that saw that had seen him notch up a seven-shot victory in last week's Wells Fargo Championship at Quail Hollow.
And the 26-year-old's show of anger, which saw his 3-wood thrown after a poor shot on the 17th, summed up his patchy day.
More frustration was to follow as he tossed his ball into the water after missing a birdie putt on the 18th and being forced to settle for par.
The PGA Tour website quoted McIlroy -- who could face a European Tour fine for throwing a club -- as saying: "Physically I'm all right. I am feeling myself getting a bit angry out there, which I haven't been doing over the last few weeks.
"I just need to stay in control of my emotions because if I'm a little tired, a little fatigued mentally, I'll start to be hard on myself and start to get down on myself.
"I have to try not to do that over the next few days. I'll just try and get as much rest as I've been getting and I should be OK.
"I need to keep everything on an even keel out there."
After throwing his 3-iron into the water in March, McIlroy said: "I just let frustration get the better of me. It was heat of the moment, and if it had been any other club I probably wouldn't have done.
"But I didn't need a 3-iron for the rest of the round -- so I thought: 'Why not?'