Though still troubled by a nagging back injury, the Australian fired a eight-under 62 – the low round of the day – at the TPC River Highlands course.

His masterful bogey-free round on Friday was seven shots better than his opening 69, which gave him a one-shot lead at nine under over two-time Masters champion Bubba Watson and first-round co-leader Kramer Hickok.

It ended a tough run for Day, who was in discomfort again on Friday from the back injury that forced his withdrawal from the Memorial and contributed to him missing out on a spot in the US Open.

"Sometimes when you do have sort of an injury or stiffness, even if you're sick, sometimes you can come out and play some good golf," he said.

"I was fortunate enough to not really get in my own way today. Hit a lot of good quality drives and my tee-to-green was pretty solid, I thought. Then holed a lot of crucial putts out there."

Day has a new Spider putter and is noticing the difference.

"It's nice to be able to visualise a ball going in the hole again instead of thinking, 'am I going to hole this putt'? So nice to be able to do that."

The Queenslander, who has slipped to No.71 in the world rankings, is searching for his first PGA Tour win since the 2018 Wells Fargo Championship.

Applying pressure are Watson and Hickok, who are a shot ahead of a posse of seven at seven under, including Englishman Justin Rose.

Watson, a three-time winner of the tournament, had an eventful second hole where the left-hander snapped his driver mid swing before making birdie.

"It was a perfect tee shot right down the middle. Chipped it in there and made the putt for birdie," Watson said. "Ho-hum."

Cameron Smith (68) and Marc Leishman (66) are also in contention at five under.

Leishman blitzed the first nine, making five birdies en route to a 30 before making a lone bogey on the 17th.

Fellow countrymen Cameron Percy and Matt Jones are in a tie for 35th, one shot back after both made 66s.

Lucas Herbert (67) and Adam Scott (69) are three and two under respectively, with the Novacastrian only just making the cut.