PORTRUSH, Northern Ireland -- Shane Lowry said he was not pleased with the state of his game as he made the 180-mile drive north from his home town of Clara, Ireland, for this week's Open Championship.

He changed his mind set after a conversation with his caddie at a local pub, and now is 18 holes away from claiming not only his first major title, but the first Open Championship held at Northern Ireland's Royal Portrush in 68 years.

Lowry tied the second-lowest score in major championship history with his 8-under-par 63 on Saturday, July 20, narrowly missing a fourth consecutive birdie putt on the 18th hole to tie Branden Grace's 62 at Royal Birkdale two years ago.

As it stands, Lowry is 16-under par and will enter the final round with a four-shot lead over England's Tommy Fleetwood.

"My mind's a bit fuzzy at the minute," Lowry told NBC after completing his bogey-free round. "Obviously, I just had an incredible day on the golf course. I'm looking forward to sitting down a little later on and just kind of reflect on it."

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Lowry leads the field with 19 birdies this week. The world's 33rd-ranked player also leads in greens in regulation and his 197 is the lowest 54-hole score in Open Championship history. The only lower 54-hole score in any major was David Toms' 196 at the PGA Championship at the Atlanta Athletic Club in 2001.

While his game is certainly in excellent shape, the 32-year-old knows the mental part will be the biggest factor on Sunday.

Lowry held a four-shot lead after 54 holes of the 2016 U.S. Open at Oakmont, but settled for a tie for second after imploding with a final-round 76.

"Look, I'm very excited for tomorrow," he said. "Four ahead for an Open Championship in Ireland ... I don't know what to say. I don't know what to say."

J.B. Holmes birdied the 18th hole to get to 10-under par and sits alone in third place.

Four-time major champion Brooks Koepka is tied for fourth along with England's Justin Rose at 9 under. They are seven shots behind Lowry but still in contention with wind and rain in Sunday's forecast.

"It's going to be an advantage for me, with the fact that I feel like I'm striking the ball well and I'm so far back," Koepka told NBC Sports. "If you're going to have difficult conditions, it's going to be windy, maybe a little bit of rain, that's kind of what I need. I need a chance, and hopefully that will be to my benefit."

Koepka shot a 4-under 67 on Saturday despite struggling with his putter to post his 18th sub-par round in his last 20 rounds in majors. He is a combined 56 under in those rounds. Koepka has 26 major rounds in the 60s since 2017 -- five better than anyone else (Rickie Fowler and Jordan Spieth).

"Just relying on ball striking, that's it," Koepka said. "I don't think anybody in the field has hit it better than me. I've just probably putted the worst in the entire field, which is quite frustrating. I feel like I haven't made anything outside 4 feet."

Fowler posted a 66 on Saturday and is another shot off the pace, tied for sixth place with England's Lee Westwood at 8 under.

Making up eight shots on Lowry figures to be difficult. Lowry said he was aware of Sunday's forecast as he made a final push to reel off three consecutive birdies on No. 15-17 to increase his cushion over Fleetwood.

"I know it's a bad weather forecast, so I said to myself if I can make one or two coming in I'll really help myself," Lowry said. "And that's what I did, which is nice."

With the partisan crowd heartily cheering his name, Lowry was quick to acknowledge that he will need to stay mentally sharp on Sunday.

"It's going to be a difficult 24 hours," he said. "But ... God, there's nowhere else I'd rather be. Hopefully I can stick to my game plan and stick to my mental process over the next 24 hours and see where it leaves me this time tomorrow."