A poor start to the back nine at Whistling Straits has seen overnight leader Dustin Johnson drop four shots off the pace at the US PGA Championship.

Johnson bogeyed the 11th, 13th and 14th holes in Wisconsin to slump to five under on Friday, with Australians Jason Day and Matt Jones (both nine under) leading the way in the latter stages of the second round.

Earlier, Johnson had taken the lead after a birdie on the par-four 10th - his second in a row and third of the round - but his previous bad luck at Whistling Straits seemed to return on the 13th as his second shot found the rough just ahead of the green.

It left the 31-year-old with a tricky chip onto the green that Johnson landed long and after missing his par attempt, he was left with a tap-in for bogey.

Another bogey followed on the 14th.

In 2010, Johnson missed out on the play-off in Sheboygan after he grounded his club in sand on the 72nd hole, resulting in a two-stroke penalty.

Meanwhile, Day - Johnson's playing partner - knocked in his third consecutive birdie on the 13th to sit alongside compatriot Jones (through 11) at the top of the leaderboard.