Brooks Koepka reads his putt on the seventh hole during the first round of the 102nd PGA Championship at TPC Harding Park on August 6, 2020 in San Francisco, California. (Photo by Darren Carroll/PGA of America)

Brooks Koepka defiantly threw down the gauntlet to his major rivals in his bid for a hat-trick of PGA Championship victories as Shane Lowry made a bright start in San Francisco.

The big-hitting Floridian said before the start that TPC Harding Park was “a big boys” arena and having won four of his last 10 majors and finished second in two more, he made a major statement by taking advantage of excellent conditions to open with a four-under-par 66.

Mental toughness and ball-striking are the strengths of Koepka’s game and he was close to his best yesterday, hitting 14 greens in regulation to lurk just one stroke behind leaders Jason Day, the 2015 champion, who fired bogey-free 65 before being joined at the top late in the day by Brendon Todd.

They lead by a shot from a nine-man group that features the heavily-tipped Xander Schauffele, his American compatriot Bud CauleyFtenchman Mike Lorenzo Vera, Brendan Steele and major winners Koepka, Zach Johnson, Martin Kaymer and Justin Rose.

Rory McIlroy battled back from a slow start and birdied the difficult ninth for a level par 70 that will give him heart.

But it was also an encouraging day for Open champion Lowry, who opened his account with a two-under 68 to sit alongside 15-time major winner Tiger Woods, who produced a superb putting display to keep alive his hopes of a 16th major win.

“I was going out there expecting it to be quite difficult,” said Lowry who three-putted the 14th but birdied the 17th to go out in level, then birdied the first and seventh on the way home despite making little on the greens.

“I was probably a bit negative for the first few holes, but it was quite easy out there this morning. It was like anything under par would have been a great score, but today it was quite gettable this morning, and you can see that from the leaderboard.

Rory McIlroy of Northern Ireland hits his tee shot on the 10th hole during the first round of the 102nd PGA Championship at TPC Harding Park on August 6, 2020 in San Francisco, California. (Photo by Darren Carroll/PGA of America)

"I missed quite a few chances middle of the round, hitting good putts, burning the edge a little bit. It was getting frustrating, but it was nice to birdie the seventh, my 16th and get it in in two-under.

”So it was nice to play the last three holes in one-under and shoot 68. I'm pretty happy. I don't feel like I drove the ball well at all.

“I'm going to go to the range and hit a few now, but other than that I feel like my game was good today and I'm pretty happy with my score.”

Gary Woodland posted a 67 but Lowry was impressed by his other playing partner, defending champion Koepka, who turned in three-under-par, bogeyed the first but was soon back on the birdie trail, picking up birdies at the second and the par-five fourth before making a 10 footer for par at the 17th.

“He played lovely today,” Lowry said. “He's got a different driver in the bag this week, and you'd know that. He drove it long and straight, and he has that little fade back. He'll be hard to beat this week, I'd say.”

Koepka was pleased with his opening effort and ominously for his rivals, believes he can do better as he bids to become the first man to win three majors in a row since Peter Thomson in 1956.

“I feel good. I feel confident,” said Koepka, who went back to his old TaylorMade driver. “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.”

As for his bid for a hat-trick of wins, he said: “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. To do it here, it would be special. I think there's, what, six guys that have ever won three in a row. Yeah, 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.”

Day made five birdies in a bogey-free effort and it was clear that driving was key to his score.

“I drove all the ball extremely well,” Day said. “When I did miss a fairway I left myself in position to make the green and if I missed a green, I left myself a relatively easy up and down.”

McIlroy birdied the 10th but then bogeyed three holes in a row from the 12th to slip to two-over before fighting back to shoot level par.

The Holywood star, who has struggled to get off to fast starts in 15 of his last 19 majors, smothered his tee shot 40 yards left into the hazard at the 483-yard 14th but fought back valiantly around turn by making four threes in a row, three of them for birdies.

He hit just six fairways all day but followed a bogey at the sixth with a birdie at the ninth to match par.

“I need to hit a few more fairways tomorrow to try to attack some pins and get it closer, and not leave myself 4- and 5-footers for par all day,” McIlroy said.

“It was there for the taking today. I mean, I feel like I definitely could have been a few shots lower, but I grinded it out and with how I hit it on the back nine, even-par actually wasn't too bad.”

As for Koepka’s hat-trick bid, he said: "To win two majors in a row, to defend a title is impressive. So to win three major championships back-to-back, and the run of golf he's played in the majors has been incredible.

”He seems to, as he said, find his comfort zone in these tournaments, in these environments, for whatever reason that is. I think we are all just lucky that he doesn't find it every other week.”

Bryson DeChambeau threatened to catch Day when he raced to four-under-par through 10 holes despite breaking his driver as he leaned on it when bending down to pick up his tee at the eighth.

Bryson DeChambeau of the United States hands his broken driver to caddie Tim Tucker on the seventh hole during the first round of the 2020 PGA Championship at TPC Harding Park on August 06, 2020 in San Francisco, California. (Photo by Christian Pete… — Bryson DeChambeau of the United States hands his broken driver to caddie Tim Tucker on the seventh hole during the first round of the 2020 PGA Championship at TPC Harding Park on August 06, 2020 in San Francisco, California. (Photo by Christian Petersen/PGA of America/PGA of America via Getty Images)

He replaced the shaft on the ninth and made birdie there to turn in three-under, then birdied the 10th.

But he couldn’t keep it going in blustery afternoon conditions and signed for a two-under 68 to end the day tied for 20th.

Kaymer, 35, putted great, which helped him to a terrific start, shooting 4-under 66.

Having not played much and having missed the cut at last week’s Barracuda Championship, played opposite the PGA Tour’s WGC event in Memphis, Kaymer said he teed it up Thursday with pretty low expectations.

On the eve of the tournament, feeling a little melancholy, he decided to go on YouTube and watch the final nine holes of his 2014 U.S. Open victory at Pinehurst No. 2, which delivered good vibes.

He said he hadn’t seen it in four or five years, but watching it made him feel good. During the pandemic, he also watched the wild finish of the 2010 PGA at Whistling Straits, which he won in a playoff over Bubba Watson.

“So I like to go back to those moments,” he said. “I know you shouldn't live in the past, but if the past can help you in the present moment, I'll take it.”

As for Woods, his 68 was his lowest opening round at the PGA since 2009 (67), when he finished second.

He made five birdies against three bogeys. Of note, Woods put a new putter in his bag. He says the Scotty Cameron Newport 2 GSS putter he has used for 14 of his 15 major victories needs to be benched every now and again, and this week he has a similar Scotty Cameron prototype putter with weights on the bottom of the sole that can be adjusted to react to different green speeds.

The putter he has used with success for years is 35.25 inches in length, and this one is longer, though he didn’t specify the length.

“He's got a little more length on there, and that's just so he can practice a little bit more without back pain,” explained his friend Steve Stricker, who played nine holes with Woods on Wednesday. Woods had 28 putts in his opening round, including a 33-footer made at 13 (his fourth hole of the round) and a 20-footer for birdie at 18.

“Most of the guys on the Champions Tour have gone to longer putters as they have gotten older, because it's easier to bend over, or not bend over,” Woods said. “And so this putter is just a little bit longer and I've been able to spend a little bit more time putting.

“It's also very similar to my sand wedge. I putt with my sand wedge all the time at home. I like the feel in my right hand. I like blading putts and hitting the ball in the equator.”

Graeme McDowell was tied 90th after a two-over 72. The 2010 US Open champion bogeyed the eighth and ninth to turn in two-over before playing the back nine in level par with birdies at the 10th and 17th cancelled out by bogeys at the 13th and 16th.

McDowell was 132nd for driving accuracy ((35/7 %) and 114th for greens in regulation (50%).