MEMPHIS (Tennessee) • Brooks Koepka has struggled in his last three tournaments, missing the cut in two of them, while enduring pain in his surgically repaired left knee.

But the former world No. 1 might be coming into form ahead of next week's PGA Championship at TPC Harding in San Francisco, where he will be defending his Major title.

The four-time Major champion got his defence of the WGC-FedEx St Jude Invitational crown off to a flying start, nailing birdies on his first four holes on Thursday.

He was smoking out of the gate and did not let up throughout the opening round to close with an eight-under 62, two strokes clear of fellow Americans Rickie Fowler and Brendon Todd. He is also eight in front of new world No. 1, Jon Rahm of Spain.

On matching his career-low round on the PGA Tour, the 30-year-old attributed his improvement - after several dismal displays since the circuit's restart in June - to being reunited with his coach Pete Cowen.

Before the Memphis event, the duo had not been able to meet due to the coronavirus pandemic and Koepka revealed he was thankful for the tough love after over four months of separation.

"With Pete, it's more psychological," he said. "He'll beat me down, tell me I can't do something, he'll jump on me pretty good and I enjoy that when someone tells me I can't do something.

"Telling me I'm not going to win, it will be awhile... I appreciate everything that Pete does, even though he is chewing me out.

"I drove it well, putted well, chipped it well. I did everything very solid. I never really felt like I was having to work too hard out there, so that's a little different than it has been.

"The confidence has been there. It is a matter of going out and doing it. It is going to come around. It feels like I am back to normal, so it is good to see a good result."

Since suffering a torn patella tendon last year, Koepka has been undergoing stem-cell treatment while opting to put off surgery.

Choosing not to go under the knife has meant hours on the greens tinkering with the mechanics of his swing and his leg positioning, and he is hopeful of recovering the form that has seen him win successive PGA of America Player of the Year awards.

"We put in a lot of work and found something by making a little adjustment in my set-up - where my weight distribution was going," he added.

"It is clicking and it feels good. It feels back to normal. I feel way more in control."

Like the remainder of the PGA Tour calendar, the PGA Championship will be held behind closed doors.

Organisers confirmed on Thursday that former Major champions Francesco Molinari of Italy and Ireland's Padraig Harrington will not be part of the field.

The pair, who have not played on the PGA Tour since its resumption, will be replaced by Americans Troy Merritt and Talor Gooch.

AGENCE FRANCE-PRESSE, REUTERS