The Australian, who won this tournament in 2015, woke up with a share of the lead, lost it to China’s Haotong Li before his tee time, regained it with three early birdies, and then lost it again with some travails at the 12th hole.

But this was a day for holding ground and Day, 32, did it nicely with his second-round 69, one-under par.

At six-under overall, he is tied-second behind Li at eight-under going into the weekend after the Chinese player, who played the Presidents Cup in Melbourne late last year, shot a dazzling bogey-free 65 today.

Englishman Tommy Fleetwood posted the day’s low round, a 64, to join Day in second place along with duel defending champion Brooks Koepka, another American Daniel Berger, England’s Justin Rose and France’s Mike Lorenzo-Vera.

TPC Harding Park played much tougher overall, although Day relished it.

“It kind of separates (the field),” he said afterward. “If you have an easy golf course, everyone’s in play.

“With a difficult golf course, the good players who are playing well usually get to the top. I would much prefer playing a difficult golf course and I know we’ll have a difficult golf course going into the weekend. I asked for that, then when I played it, I’m stressed out. But I’m going to enjoy the weekend.’’

Day hit it close on the fourth, fifth and seventh holes for birdies that vaulted him back to the top of the leaderboard.

But his troubles began at the par-four ninth, where he pulled his drive up against a boundary fence, was lucky to get a line-of-sight free drop, but made bogey.

Then after a nice bounce-back birdie at the par-five 10th where he hit the green in two, he made a double bogey six at the par-four 12th hole, one of the most difficult on the course today, after missing the fairway left, pushing his approach right, then taking two chips and two putts to get down.

Day parred his way in from there, leaving great birdie opportunities at the 16th and 17th a few centimetres short of the hole.

“They’re starting to get that purple look to them,’’ he said of the Harding Park greens.

“It’s like a dry, purple look. Once you see that, balls start to bounce on the greens and they get really quick. Once you have that and match it with the winds we had – we had a lot strong winds yesterday, but there were gusts out there – it was difficult.”

Day came in on the back of three straight top-10 finishes and does not appear to have been bothered too much by his back injury this week. He is searching for his first win in two years.

His issues on the back nine were not unusual today. Tiger Woods could not hole a putt of substance until he bombed one on the 16th, and ultimately shot two-over par 72.

Rory McIlroy made a strong run of birdies but took a calamitous triple bogey at the 12th on his way to a 69 that could have been so much better. World No. 1 Jon Rahm (one-under) has been quiet.

Meanwhile Adam Scott was the best of the other Australians, cobbling an even-par 70 to remain at two-under and just outside the top 20 at his first start since the interruption caused by the Covid-19 virus this year.

Queenslander Cameron Smith (69 today) also played well enough to get through to the weekend in 45th place.

Marc Leishman finished two-over after shooting 70-72 to miss the cut narrowly, while Lucas Herbert (three-over) and Matt Jones (five-over) will not make the weekend’s play in San Francisco.