Backing up from his drought-breaking victory at the AT&T Byron Nelson Championship, Day struggled to a six-over-par 76. Day trails clubhouse leader and first-round playing partner Bryson DeChambeau by 10 shots, while Eric Cole was one shot better at the top when darkness caused play to be suspended on his 15th hole.

Day amassed a 23-under winning total last week in Texas but couldn't manage a single birdie on Thursday after frost delayed the start of play at the season's second major championship by almost two hours.

The former World No.1 and 2015 PGA championship winner opted against playing a practice round at Oak Hill, preferring to save his energy, and now needs all he can muster on Friday just to make the halfway cut.

Starting on the 10th, Day's troubles began when his tee shot flew out of bounds on the 14th and wound up with a double-bogey six.

He added four bogeys to end his round, languishing in a tie for 130th spot in the 156-player field.

Australia's other early starters, Open champion Cameron Smith mixed three birdies with five bogeys in a two-over 72, while Min Woo Lee posted a 73.

Out in the afternoon, Cam Davis signed for an eventful one-over 71 that included a double bogey at the 17th, while Herbert had eight bogeys in his 75 and David Micheluzzi was getting a stern introduction to major championship golf.

The PGA Tour of Australasia Order of Merit winner was seven-over through 15 holes when play was suspended.

Meanwhile, Adam Scott stood up as the best of the Aussies by some margin, with the Queenslander tied with DeChambeau at four-under as he played the 18th hole.

A drive in the middle of the fairway was followed by an approach that found the greenside bunker at the final hole. Scott's first effort to extricate himself from the sand was unsuccessful, the second shot only just reaching the putting surface. A bogey putt left short saw a double bogey and two-under round of 68 to put the Aussie in a share of sixth.