Jason Day played some of his best golf of the season at Firestone Country Club today but an apparent flare-up of his back injury put a dampener on his improvement.

Day, who has been in a slump by his high standards, made a couple of bomb birdie putts and had a chip-in from deep rough on his way to a four-under par 66 in the World Golf Championship’s Bridgestone Invitational in Ohio.

But the 29-year-old Australian winced and grimaced throughout his round and his creeping walk indicated that he was suffering from back pain again, likely for the first time this year.

Day missed the last three months of 2016 because of his back injury, diagnosed as a ligament strain in the lower back, so the recurrence of the injury is less than good news. After his round, he told reporters at Firestone that he was not seriously injured, moreso stiff in the back.

Has not won since the Players Championship in May, 2016, when he was No. 1 in the world, and his world ranking has dipped to seventh.

A long day at Firestone did not help his cause, with several weather interruptions, but Day began with three consecutive birdies with long putts – seven metres at the first, five metres at the second and eight metres at the third. At the par-four seventh hole he found a greenside bunker, flopped out but did not reach the green, then chipped in from the deep greenside rough for par.

Day kept pounding the ball, was wild off the tee but good around the greens, and in the end he was just four shots from the lead held by America’s Jimmy Walker (68-65) at seven-under par.

Adam Scott and Scott Hend are the other Australians in the mix through two rounds of the $US9.75 million tournament, which precedes the PGA Championship in North Carolina. Both Queenslanders are at one-under in a tie for 12th.

Rod Pampling, Sam Brazel and Marc Leishman are well off the pace.

It was a decent day for Australians, with veteran Stuart Appleby leading the Barracuda Championship in Reno after two rounds, threatening to conjure a miracle comeback from several years of back trouble.

Appleby, who has limited status on tour, has 24 points in the modified stableford event to lead by one point.

At the Women’s British Open at Kingsbarns, Katherine Hull is the leading Australian at two-under par through two rounds.

In-Kyung Kim of South Korea leads at 11-under.

Su Oh of Victoria was the only other Aussie to make the cut.