Miami - World No 1 Rory McIlroy's frustration spectacularly boiled over on Friday as he hurled his three-iron into the water after plunking a shot into a pond at Doral.
McIlroy, who will try to complete a career Grand Slam and win his third major title in a row at next month's Masters, has been off to a shaky start in his final push to Augusta National.
He opened his European Tour campaign with a runner-up finish and a win last month in Dubai, but missed the cut in his 2015 US PGA Tour debut at the Honda Classic before teeing it up in this elite World Golf Championships event.
After a first-round 73, McIlroy opened with back-to-back birdies on Friday, but his tee shot at the third found a watery grave and he missed a five-foot par putt at the par-3 fourth, although a 24-foot birdie putt at the seventh appeared to right the ship.
But at the par-5 eighth, McIlroy's second shot sailed left and splashed down into the water, where it was soon joined by McIlroy's club, which spun like a helicopter until vanishing into the briny deep.
"Frustration got the better of me," McIlroy said. "I've sort of been fighting that miss basically for the last couple of weeks."
McIlroy stressed he didn't think the behaviour - which is rare for the Northern Irishman - was anything to be proud of.
"It felt good at the time, but right now I regret it," he said. "I walked away with a bogey and I was able to regroup and shoot a decent score."
After swallowing his bogey at the eighth, McIlroy opened the back nine with a birdie and picked up another at the par-5 12th.
He found a greenside bunker at 14 and missed another five-footer for par on the way to a bogey, but bounced back at 17, sinking a 21-foot birdie putt before closing with a par.
His one-under 70 left him tied for 11th on one-under 143, eight shots behind second-round leader J.B. Holmes.
McIlroy may have been feeling sheepish about his petulant gesture, but popular pro John Daly, a two-time major winner playing this week in the US PGA Tour event in Puerto Rico, took to Twitter to offer light-hearted support.
"@McIlroyRory, ur #1 in golf & I am #1 in club throwing - only 798 throws away from catching me on the PGA tour! #showsucare," Daly tweeted.
While McIlroy said last week as he prepared for the Honda Open that he was eager to shoulder the burden of being golf's man to beat, the episode is sure to spark questions over whether he's feeling the pressure of his high profile.
Golf Channel reported McIlroy will play Augusta National next week with a celebrity playing partner - New England Patriots quarterback Tom Brady, who won his fourth Super Bowl title last month.
Meanwhile, JB Holmes hit four balls into the water but emerged with a one-over 73 that saw his lead slashed in half after two rounds of the WGC-Cadillac Championship.
After a stunning 62 on Thursday, Holmes had a 36-hole total of nine-under-par 135, his four-shot overnight lead over Ryan Moore whittled to two strokes over his fellow American.
Leading second-round scores on Friday at the World Golf Championship Cadillac Championship at Doral in Florida (USA unless noted, par-72):
135 - J.B. Holmes 62-73
137 - Ryan Moore 66-71
138 - Adam Scott (AUS) 70-68
140 - Bubba Watson 71-69, Henrik Stenson (SWE) 69-71
141 - Dustin Johnson 68-73, Alexander Levy (FRA) 68-73, Ryan Palmer 71-70
142 - Jamie Donaldson (WAL) 70-72, Sergio Garcia (ESP) 73-69
143 - Webb Simpson 74-69, Jim Furyk 70-73, Lee Westwood (ENG) 71-72, Charl Schwartzel (RSA) 71-72, Brooks Koepka 69-74, Rory McIlroy (NIR) 73-70, John Senden (AUS) 73-70
144 - Charley Hoffman 70-74, Morgan Hoffmann 73-71, Gary Woodland 70-74, Patrick Reed 71-73, Jordan Spieth 75-69
145 - Shane Lowry (IRL) 71-74, Victor Dubuisson (FRA) 72-73, Louis Oosthuizen (RSA) 71-74, Rickie Fowler 68-77, Brendon Todd 72-73, Kevin Na 74-71
146 - Bae Sang-Moon (KOR) 75-71, Graeme McDowell (NIR) 73-73