(CNN) -- World No. 1 Rory McIlroy lost his cool in the midst of a heatwave at the BMW PGA Championship as Luke Donald stepped up his challenge to usurp the 2011 U.S. Open champion at the top of the rankings.
McIlroy, from Northern Ireland, struggled to a two-over-par round of 74 on the opening day at the Wentworth course while Donald, the defending champion, sits within striking distance of the leaders on four-under.
Donald can regain the No. 1 spot with victory at the tournament and must finish in the top eight to have a chance of overtaking McIlroy. His six birdies helped him into a tie for eighth place.
Scotland's David Drysdale and Ireland's Peter Lawrie lead on six-under after day one with England's Justin Rose and Alavaro Quiros, from Spain, in a group of five players one shot back.
McIlroy's round included a run of four bogeys in five holes and at one stage he hurled a club to the floor in frustration after a poor shot.
"It's a bit of deja vu from last year," he told reporters. "I kept getting off to good starts and sort of making a few bogeys around the turn and not getting any momentum back.
"Two under through seven, feel like I'm playing well, then I make four bogeys in five holes. It's pretty disappointing and I feel like I'm playing pretty well, I just need to go out there and shoot a score."
McIlroy's compatriot Graeme McDowell's round turned sour shortly after he'd walked off the 18th green as his ball was shown to have moved prior to hitting his second shot from the rough on the closing hole.
The 2010 U.S. Open champion was unaware the ball had moved and he was penalised two strokes -- one for not replacing the ball into its original position and one for striking it from the wrong place.
It turned his round of level par into a round of two-over. He told reporters: "How are you supposed to attempt to place the ball when you're not sure it's moved in the first place? It's just a harsh one -- one of those freak scenarios in golf.
"It's my fault. I probably should have called for a referee. That would have saved me one shot, but what are you supposed to do -- the whole area was sort of bouncing.
"I was aware of it and asked for it to be looked at. The rules are there for everybody's protection."
Three-time major champion Ernie Els, from South Africa, is on four-under and compatriot Charl Schwartzel, the 2011 Masters champion, finished on three-under.
Former world No. 1 Lee Westwood, who was beaten by Donald in a playoff at last year's tournament, birdied the last to finish two-under.