Brooks Koepka said he is feeling confident in his bid for a US PGA Championship three-peat after shooting a first-round 66 in San Francisco.

Koepka impressed in the opening round of the year's first major due to coronavirus, the two-time defending champion holing six birdies and two bogeys to be just a shot off the pace at TPC Harding Park.

Asked what it would mean to win three straight US PGA titles, American star Koepka replied: "It would mean extra because I wasn't able to do it at the U.S. Open.

"I think that drove me nuts a little bit. I mean, obviously I played about as good – I played good golf, but I just got beat by Gary [Woodland].

"To do it here, it would be special. I think – I don't know how many – I think there's, what, six guys that have ever won three in a row. Not a bad list to be on. That's the whole goal every time we tee it up in a major is to win them. The whole year is spent prepping for these four."

Koepka started on the back nine and after bogeying his second hole, the four-time major champion responded by birdieing four of his next seven.

The big-hitting 30-year-old dropped the first following the turn, however, a pair of birdies on the second and fourth holes ensured he ended the day at four under.

"I mean, it's only 18 holes right now. I feel good. I feel confident," Koepka told reporters afterwards.

"I'm excited for the next three days. I think I can definitely play a lot better, and just need to tidy a few things up, and we'll be there come Sunday on the back nine."

Koepka entered the US PGA having fallen short in his title defence at the WGC-FedEx St. Jude Invitational, though it marked his best result of the year and just his second top-10 finish in 11 events.

"I feel right where I should be," he said. "If you would have said last week and this week, I'd feel perfect, right where I need to be. I'm excited. I'm ready to play. But you asked for three weeks, so it's been okay. There was a missed cut in there."

Koepka added: "The majors almost seem like an easier week for me, nine holes pretty much every day in the practice rounds and try to stay off my feet and not do too much.

"I think sometimes guys cannot overprepare, but just practice a little too much. If you're going to be here until Sunday it's pretty mentally draining. Physically it's fine, but mentally I'm done after four days of this."