Defending champion, Koepka, leads by one over New Zealand’s Danny Lee, his bogey free PGA record equalling round of 63 over one of the more demanding layouts in the US adding further to his established reputation of handling the tough golf courses so well.
“This golf course is brutal,” said the leader. “If you're not going to drive it, like I said, it tests every aspect of your game. You've got to drive the ball straight. It's long, so you've got to hit it far and really position yourself with some of these shots in.
“You can't miss -- you can't take a shot off, and that's what I love. I think that's why I play so well at U.S. Opens, this golf course, typical U.S. Open golf course. I mean, if you don't have a good day, you can very easily shoot 5, 6-over.
"There's a fine line between 5, 6-over and a couple under out here. It's a fun golf course to play, that's for sure.”
Day was over par early and when he bogeyed the 10th he had slipped to 2 over. Then would come a fightback. He birdied the 11th and 12th and when he birdied the 16th he was under par for the first occasion on day one.
Day then missed a very good chance for another birdie at the 17th after his tee shot finished 6 feet from the hole and parred the last for his under par effort.
“It is the first day and we just have to be patient with ourselves,” said Day. “There’s a lot of golf to be played. You could be 2 or 3 over today and still have a good shot at the end of Sunday.
“It is crucial to get yourself in the fairway here. I can’t say it enough as once you are in the rough it is pretty much layup, wedge it on and hole a putt that way.
There has been concern over Day's ongoing injury issues but he would say that things were ok today although he is wary of it especially in the cooler weather.
"I feel good. I felt like my back just tightened up a little bit kind of on 15, so I just put this on to try and warm it up a little bit. Obviously it's starting to cool down now, so I've just got to keep that in mind."
Of the other Australians, Adam Scott finished at 1 over, a double bogey at the 8th hole (his 17th) a disappointing way to finish off what had been a solid start until that point, especially given he also began his round with a bogey several hours earlier.
Scott is, though, just four shots out of the third place Tommy Fleetwood, although is eight shots from Koepka.
The next best of the eight Australians in the field were Cameron Smith at 3 over 73, and Marc Leishman and Lucas Herbert 74, Brendan Jones 78 and Craig Hocknull and Stuart Deane 82.
Danny Lee played in the afternoon field and his eight birdies highlighted just how well he played. His season to date in 2019 has given no clue that such an impressive start was imminent, a 7th place at the Texas Open his only finish inside the top 30 this year thus far.
Lee does not enjoy a good record in the majors but he talked today how increased distance from the tee in recent times is working in his favour.
“Well, at first, I wasn't hitting it far enough to compete out here on the major championship -- PGA Championship or U.S. Open,” said the 28 year old former US Amateur Champion.
At the Masters, I might have had a chance. But now I'm definitely hitting it further. I can carry my driver about 290, 295 in the air. That's a huge bonus for me. And that was actually the first time I actually got to play in a major with this distance.
“I know I work really hard on getting my body strong because I always wanted to hit it further. I always wanted to carry -- able to carry 290 bunker, straight over it, but I never had that until this year.”
16 players finished the day under par with 40 at even par or better.
Conditions tomorrow are expected to be more demanding than was the case on day one with winds expected to be considerably stronger. If that is to be the case then a cut score as high as 6 or 7 over is a possibility.
Much of the focus on day two, however, be on Koepka who is targeting his 4th major championship in his last eight attempts although he may well be faced with the most difficult of tomorrow's conditions given his afternoon tee time.
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