Australians Jason Day and Matt Jones were humming along nicely at the US PGA Championship until thunderstorms cut Friday's action short.

The second round at Whistling Straits was building to an impressive crescendo in Sheboygan, Wisconsin, with the likes of Day, Jones and Justin Rose surged, while overnight leader Dustin Johnson was struggling.

But all of that came to a screeching halt when thunderstorms rolled in, ending Friday's action prematurely.

Day and Jones sit tied for the lead at nine-under-par.

The former was on the par-four 15th hole when the siren went off just after he hit his second shot. Jones, starting on the back nine, had just hit his tee shot on the fourth hole, so he will have some work left in the morning as well.

Rose had just birdied the par-three 17th to get to eight under and in third place by himself.

PGA Tour rookie Tony Finau was six under through 13 holes, taking him to seven under overall and just two shots back.

Finau has been one of the top rookies on the PGA Tour this season, but is still searching for a breakout win.

Johnson started the day with the lead after shooting a six-under 66 on Thursday but he struggled as the second round went on.

The 31-year-old slumped to five under through 14 after reaching eight under at one point, until he bogeyed three of his last four holes before play was suspended.

Thursday's afternoon wave struggled with gusts off Lake Michigan but benefited from Friday's calm. One of those players was Japan's Hiroshi Iwata, who introduced himself to the world by firing a nine-under 63 to tie a major championship record.

Iwata finished the day with eight birdies, a bogey and an eagle.

The 34-year-old had a chance to break the best-round record on the final hole, but his chip shot just veered off to the right, and he tapped in for par.

There have now been 27 rounds of 63 in major championship history, including the Masters, U.S. Open and British Open. Thirteen of those came in the PGA Championship.

Jordan Spieth shot a 67 on Friday to climb to six under.

The 2015 Masters and U.S. Open champion scrambled around the course nicely on Thursday but found things easier during round two, finishing with six birdies and a lone bogey.

The second round is set to continue on Saturday, before the third round gets underway.