Bud Cauley - tied 33rd in 2017 - finishes with a bogey, but it's four under for a first round of 66. A great round for the 30-year-old, who was home-schooled so he could practice golf during the day.

It's all in his PGA profile - don't shoot the messenger!

Image source, EPA

Former world number one Jason Day posted a five-under-par 65 to take the clubhouse lead.

The 32-year-old, who won this event in 2015, said after his round: "I drove the ball extremely well. When I did miss the fairway I gave myself good chances to get to the green.

"I am surprised at the low numbers. I thought probably high single digits would win it. We got lucky with the weather today, especially with the wind. It’s PGA week so it’s going to toughen up later in the week.

"I’m just trying to work on some feels in my swing and it’s panned out. I’ve worked extremely hard on my putting and really trusting it.

"I lost a lot of confidence with my putting and that’s slowly starting to come back"

Tiger? Bryson? Rory? A Justin?

Use #bbcgolf , externaland remember to explain the reason your choice.

Spaniard Jon Rahm is striding up to practice greens - he tees off at 21:58 BST. The world number two is wearing what looks like one of those masks a builder would wear when trying to deal with asbestos.

Mask fashion will be scrutinised this week. Largely be me.

Rickie Fowler (21:47 BST) has a yellow and sky blue mask. Standard Fowler.

McIlroy plays a lovely sand iron out of a spot of bother on the par four sixth - he's got four yards for par. Easy save.

Well, it should have been. Too much pace on that and the lip sends it away.

Bogey and back to level par coming up.

Image source, PGA Tour

Tiger Woods was in a relaxed mood at his pre-event news conference, recalling his previous visits to Harding Park.

He often played there as a junior and while at Stanford University, and on his two appearances there as a pro he won all five of his matches as USA won the Presidents Cup in 2009 having won the WGC-American Express Championship in 2005. He beat John Daly in a play-off when the "electric" atmosphere left him "half deaf".

“I guess you couldn’t have put two more crowd-drawing players together in a play-off," he said. "It was loud, people were into it and it was a lot of fun.”

The 15-time major winner added that he was enthusiastic about some of the changes he’s made to his game while playing at home, but when pressed on what he'd changed he replied: “Oh, I’m not gonna tell you that!”

Then with the final question he was asked if he could win this weekend, he said simply "of course" while staring back at the reporter for a few seconds, then he broke out into a smile and headed straight off.

Ask a stupid question...

Martin Kaymer gives his putt on the par-four ninth - the last - a bit too much oomph and drops a shot. Still it's a 66 and four under.

Aussie Jason Day, playing in the same group, goes one better after a fabulous approach. He's on five under and is the new clubhouse leader. His one and only major is the US PGA win in 2015.

Image source, EPA

Tiger Woods makes back-to-back birdies with a relatively straight 10 footer on the fifth to go to two under - he has four to play.

Yikes!

Image source, Reuters

American Bud Cauley - whose dream foursome would include Megan Fox, Carrie Underwood and Jack Nicklaus according to his PGA profile - has just birdied the 16th for back-to-backs. He is now a joint leader on the course on five under!!!

Martin Kaymer -5 after 16 holes

Xander Schauffele -5 (15)

Zach Johnson -5 (14)

Scottie Scheffler -4 (F)

Jason Day -4 (13)

Brooks Koepka -4 (14)

Bud Cauley -4 (15)

Selected others: -3 Tony Finau, Dustin Johnson, Gary Woodland; -2 Justin Rose, Tom Lewis; -1 Tiger Woods, Rory McIlroy; E Justin Thomas

Image source, Reuters

Young Dallasite Scottie, 24, has played an absolute blinder so far and is currently the clubhouse leader on -4 after a 66 which included six birdies.

He's not exactly pulled up many trees during his two-and-a-bit years as a pro, but before heading over to San Fran he played in six events post-lockdown and managed a commendable T22 at the Memorial and T15 at the St Jude.

Laugh ye not at The Scheffler.

Image source, Getty Images

Bud Cauley - a late entry - has just sunk a 100 footer (or something) for a birdie on the par four 15th. He's a shot off the lead on -4.

We've only had two birdies so far today.

The hole: A short, downhill par four which could encourage the bigger hitters to cut the corner of the right-to-left dog-leg and get close to the green.

Image source, Getty Images

Speaking of the great Walter Hagen, Brooks Koepka aims to become just the second player to have achieved a 'threepeat' in US PGA history after the 11-time major winner.

Hagen lifted the Wanamaker Trophy four years in a row from 1924, when the US PGA was a match play tournament. No player has won it three years running since it became a stroke play event in 1958.

The big hitter is only a shot behind the current leaders on four under after 13 having birded the par-five fourth.

Image source, Getty Images

The last PGA event to be held at Harding Park was the WGC-Cadillac Match Play in 2015, when Rory McIlroy beat Gary Woodland to lift the Walter Hagen Cup.

The four-time major champion was one of those who started on the 10th - which he birdied - but then made successive bogeys on the 12th, 13th and 14th. They were all 'makeable'.

He looked tentative, and we know what happens to McIlroy at majors when he's like that.

However, birdies on the 16th, 18th and first means we probably won't see one of those 80+ rounds from the man from NI.

He's currently -1 after 12 - four shots off the lead.

Image source, EPA

Image source, Getty Images

The back nine opens with a good scoring opportunity with this reachable par-five. Avoiding bunkers to the right of the fairway leaves an approach to a receptive green.

Many of the big hitters, literally and metaphorically, started on this hole - we've already seen an incredible 17 birdies here. There have only been eight birdies so far on the par-five fourth.

Image source, Reuters

Very, very hard to predict how Tiger Woods will do this week.

Asked by TV journos whether he could win the major, Woodsy responded with a succinct "yes".

The thing is he's only played one tournament in the run-up - the Memorial in July where he finished tied 40th. Hmm.

"I feel good," added Woods. "Obviously I haven't played much competitively, but I've been playing a lot at home.

"Just trying to get my way back into this part of the season. This is what I've been gearing up for. We've got a lot of big events starting from here, so looking forward to it. This is going to be a fun test for all of us."

He's currently level after 11 holes having started on the 10th, and is playing in a trio with McIlroy and numero uno Justin Thomas - who as a group have at times been more Colliers Wood than Hollywood.

Woods made two birdies in the opening four before dropping a shot on the 15th after the hole gave the ball some lip. He's just bogeyed the par four second.

USA unless stated - time in BST, local time in brackets

1527 (0727) Tony Finau, Danny Willett (Eng), Patrick Cantlay

1538 (0738) Jason Dufner, Martin Kaymer (Ger), Jason Day (Aus)

1611 (0811) Gary Woodland, Brooks Koepka, Shane Lowry (Irl)

1622 (0822) Jordan Spieth, Dustin Johnson, Justin Rose (Eng)

1633 (0833) Rory McIlroy (NIrl), Tiger Woods, Justin Thomas

2114 (1314) Kevin Na, Patrick Reed, Robert Macintyre (Sco)

2125 (1325) Bubba Watson, Matthew Wolff, Graeme McDowell (NIrl)

2136 (1336) Viktor Hovland (Nor), Tommy Fleetwood (Eng), Hideki Matsuyama (Jpn)

2147 (1347) Rickie Fowler, Bryson DeChambeau, Adam Scott (Aus)

2158 (1358) Phil Mickelson, Jon Rahm (Spa), Sergio Garcia (Spa)

2209 (1409) Paul Casey (Eng), Ian Poulter (Eng), Webb Simpson

Straight up my deep rough-ians, I want to know who you think will be celebrating in front golfers, caddies and staff on Sunday.

Tiger? Bryson? Rory? A Justin? Or will Austria's Sepp Straka (best and only major finish a T28 at the 2019 US Open) shock them all?

Use #bbcgolf , externaland remember to explain your reason.

We will get into the live action soon - it's been a great start for German Martin Kaymer who produced a monster of a monster putt on the five par fourth hole for an eagle. He started his round on the 10th so he's looking good for the clubhouse lead, although Zach Johnson and good ol' Scottie Scheffler might ensure it's just a joint lead.

And then there's Rory and Tiger...