Day and Cameron Smith are both at nine-under 201, just one shot off the pace, while another four Australians are within five shots of co-leaders Bubba Watson and Kramer Hickok.

A day after a sparkling 62 catapulted him to the top of the leaderboard, Day made a bogey on the 18th to card an even-par 70 while Smith jumped 17 spots to a tie for third with a 66.

It was clear on Saturday that Day was still struggling with ongoing back issues, which forced his withdrawal from the Memorial Tournament recently and contributed to him missing out on a spot in the US Open.

"I'm just stiff," Day said of his back.

"I had allergies out there too, so that wasn't good.

"I was on 14 and I needed some eye drops and Bubba pulled them out of his pocket, so that was nice."

Smith enjoyed a solid round at TPC River Highlands with six birdies while a bogey on the 16th means he will play in the second-to-last group on Sunday with Russell Henley (68).

"Few errors with the wind there but it was so up and down the whole day," Smith said.

Day and Smith won't be the only Australians chasing victory on Sunday with Matt Jones, Lucas Herbert, Marc Leishman and Adam Scott all in contention, with only Cameron Percy too far back to mount a serious challenge.

A roller-coaster three-under 67 has Jones at seven under, in a group sharing ninth which includes World No.2 Dustin Johnson (65).

Herbert (67), who had an eagle on the par-5 6th and Leishman (69) are a shot further back while Scott was bogey-free for a 67 that has him five under and sharing 27th.

Percy has fallen well back with a 72 leaving him two under.

For Day, who is ranked 71st, a 13th PGA Tour title and first since the Wells Fargo Championship in May 2018 would go a long way to securing a place in the top-50 and an automatic spot in the remaining major this year.

"It's a pretty critical time for me," the Queenslander said.

"It's just hard to have an injury like this or not even an injury, but have a stiff back knowing that I have to go out and play.

"So I've just got to suck it up and just do it."

Smith fancies his chances as he aims to add another PGA title this year – to go with his teams triumph alongside Leishman in April.

"I feel like I've been up there enough to really give it a crack ... probably the most confident I've been over the driver for a long time, which is a big thing for me," the World No.28 said.

Watson – a three-time winner of the tournament – fired a 68 while Hickok matched the two-times Masters champion's score but was left to rue back-to-back bogeys on the 17th and 18th holes.