Day is among a stack of big names who have set the stage for an exciting finish to the first major of a year ravaged by COVID-19.

It wasn't easy for Day, who barely held on during a lacklustre, even-par 70 on day three but, at six under, he will start Monday morning's final round just three shots off the pace.

Dustin Johnson took the 54-hole lead at San Francisco's TPC Harding Park with a superb 65 that also has him poised to capture a second major to go with his 2016 US Open title.

At nine under, big-hitting Johnson will take a one-shot lead over fellow Americans Scottie Scheffler (65) and the powerful Cameron Champ (67).

Defending champion Brooks Koepka (69), Collin Morikawa (65) and England veteran Paul Casey (68) are seven under.

Bryson DeChambeau (66), Tony Finau (67), Justin Rose (70) and Daniel Berger (70) are tied with Day at six under.

Day, the 2015 PGA champion, has not been in contention on golf's biggest stage since the 2019 Masters at Augusta National.

The 32-year-old found form recently with three top-10 results on the US PGA Tour leading into the PGA Championship.

He is hungry to go the next level.

"It's just nice to be able to be in contention at a major championship again and know that your game is good enough to get in contention," Day said.

"We haven't played a major in ages. It's strange, but I'm excited to be here."

Day is also buoyed by the chance to become Australia's first male winner of multiple majors since Greg Norman captured a second British Open title in 1993.

But Day he will try to ignore that milestone and the leaders on the final day at Harding Park.

"I won't focus on the lead at all," he said.

"If I can walk off at the end of the day knowing I gave everything, then I'll be happy. Hopefully it's enough to win."

Day faces a stern challenge in reeling in Johnson, who bounced back from a double-bogey at the ninth with four back-nine birdies.

Johnson's world-class driving is tailor-made for major championships, having finished runner-up in the Masters (2019), British Open (2011) and last year's PGA.

"I definitely have experience in this situation that definitely will help tomorrow," Johnson said.

But four-time major winner Koepka, seeking to win a third successive PGA Championship, questioned Johnson's ability to close.

"I like my chances; when I've been in this position before, I've capitalised," Koepka said of Johnson. "He's only won one."

Australian world No.9 Adam Scott squandered a chance to make ground when a flat, even-par 70 left him at two under while countryman Cameron Smith (70) finished at even par.

Four-time PGA champion Tiger Woods (72, two over) is well out of contention for a 16th major championship win.

Australian Associated Press