ST. LOUIS, MO - August 11: Shane Lowry hits his tee shot on the 13th hole during the third round of the 100th PGA Championship held at Bellerive Golf Club on August 11, 2018 in St. Louis, Missouri. (Photo by Montana Pritchard/PGA of America)

Rory McIlroy's major drought will slip past the four-year mark, but Shane Lowry will have a chance to get his first after he birdied the last for a grinding 69 to go into the final round of the US PGA Championship just four shots behind US Open champions Brooks Koepka.

The affable Offaly man has emerged from his mini-slump in sensational fashion over the past three weeks, and with his confidence on a high, he polished off his delayed second round superbly to post a six-under 64, then dug deep at the end of a 14 hour day to knock in a 10 footer at the 18th at Bellerive Country Club.

Koepka looked ready to leave the field spluttering in his exhaust fumes when he went out in 30 to lead by three on 13-under.

But he wobbled coming home, and after following back to back bogeys at the 14th and 15th, he didn't take advantage of a 12-foot eagle chance at the 17th, tentatively two-putting for birdie before parring the last for a 66 and a two-shot lead over Adam Scott.

Spain's Jon Rahm (66), Rickie Fowler (69) and Gary Woodland (71) are just three behind in third with Lowry tied for sixth on eight-under with Tiger Woods (66), Stewart Cink (66), Jason Day (67), defending champion Justin Thomas (68) and South Africa's Charl Schwartzel (69).

"I said to Alan walking down 18, make birdie here, and we have a right chance tomorrow," beamed Lowry beamed. "So that's what I did. I feel like I'm where I want to be. I said early on in the week, late single digits going into tomorrow will be good and it is.

ST. LOUIS, MO - August 11: Sane Lowry misses his putt on the 12th hole during the third round of the 100th PGA Championship held at Bellerive Golf Club on August 11, 2018 in St. Louis, Missouri. (Photo by Montana Pritchard/PGA of America)

"I'm very excited for tomorrow. It's great to have your brother on the bag, your dad out there watching, and you're going competing in the final round of a Major. It doesn't get much better. So I'm looking forward to it."

Lowry had to work hard for his 69, especially after he followed three pars with a bogey at the tough fourth, where he just missed the green and couldn't save par from nine feet.

It was a battle from there, but he birdied the dangerous, par-three sixth from 13 feet and turned in one-under, firing a 152-yard approach to nine feet before rolling in the putt.

"I ground it out well," said Lowry, who bogeyed the 14th, almost chipped in at the 16th and failed to birdie the 17th before getting his reward with that sweet three at the 18th.

"I didn't have my best. The greens were so slow. I couldn't get the pace. Felt like I had hit my putts quite lovely the first few days.

"To be honest, the one on 18, I said, let's just roll this one and see what happens. It's the first putt I didn't really hit in the afternoon and went in. So delighted to finish with that.

"If Brooks makes birdie, gets to 12, [he did] I'll only be four back tomorrow, which is a nice place to be."

Lowry does not feel he has to change his game plan tomorrow and try to be more aggressive.

"I'm not going to stand and aim at every flag tomorrow, no, because that would be just stupid," he said.

"I'm just going to go out, and when I get a number into a green where I have a good flow, then I'll go for it and try to make birdies.

"The disappointing part was not birdieing the two par 5s, especially with the 17th tee being so far up. That was the worst drive I hit all week. It's a drive where, if you hit the fairway, it's a gimme birdie. So, yeah, the par 5s, you make birdies there tomorrow and sneak a few here and there."

Lowry is fifth for fairways hit, tied ninth for greens hit in regulation and second for strokes-gained putting so he knows that more of the same and a little bit of luck will go a long way.

"I just have to play my own game and just be patient," he said of the lessons he's learned from past challenges in majors.

ST. LOUIS, MO - August 11: Brooks Koepka on the 12th hole during the third round of the 100th PGA Championship held at Bellerive Golf Club on August 11, 2018 in St. Louis, Missouri. (Photo by Montana Pritchard/PGA of America)

"Patience is everything. Yes, it is a course you make birdies, but I saw a few lads make a couple of big numbers as well.

"I didn't feel like I had my A game out there this afternoon, but I did well to shoot the score I did. And hopefully, I go out and play decent tomorrow and see what happens.

"Look, I'm right where I want to be. Of course, I'm very satisfied. I'm going to be looking forward to relaxing this evening, and late tee time tomorrow in a Sunday at a Major is where I want to be.

"I've been in this position before, so I know what it's about. Hopefully, I can go out and do a better job this time than I did the last time.

"I feel like the first two days, the two rounds, I putted very well. I'm just hitting lovely iron shots. My distance control is very good, and a couple of slip-ups on the back nine, a couple maybe there a bit tired errors, I think, more than anything else.

"But, yeah, I feel like my iron play is pretty good this week, and my distance is always good. The greens are very soft. It's easier than what it could be out there, but you still have to hit the shots, and I feel like I'm doing that this week."

With the top 20 on the leaderboard covered by six shots, experience will be key, and Lowry has plenty even if his most recent challenge was the 2016 US Open when he led by four shots going into the final round but tied for second behind Dustin Johnson.

"Look, Majors aren't easy wins. I know Brooks is used to being up there and he has two, but it's not easy winning them. There's going to be a few changes on the leaderboard tomorrow, and hopefully, I'll have something to say about it."

As for his energy levels — he's worked hard on his fitness with Robbie Cannon over the past few years — he has no fears.

"I'm good," he said. "I just played the 18 pretty flawless there. I'm obviously quite tired. It's been a long day. I'm looking forward to resting up. I've got a while before I tee off tomorrow."

ST. LOUIS, MO - August 11: Brooks Koepka hits his tee shot on the 12th hole during the third round of the 100th PGA Championship held at Bellerive Golf Club on August 11, 2018 in St. Louis, Missouri. (Photo by Montana Pritchard/PGA of America)

His watchword for the final day is simple.

He said: "Just be patient, be more patient. Wait for it to happen. Especially on this course with how soft the greens are, you can make a run and get three or four in a row, and all of a sudden, you're back in the tournament. I just need to be very patient tomorrow and wait for it to come to me."

His recent resurgence in form is no surprise given that he's played respectably all season without putting four rounds together until the RBC Canadian and the Barracuda Championship over the past fortnight.

"I came here very confident this week," he said. "I played lovely the last two weeks in Canada and Reno, and, yeah, I've seemed to just kick on.

"Obviously, my game is in decent shape, and mentally I'm in a decent place. Hopefully, I can kick on for the rest of the year, and who knows what will happen?

"If I can shoot mid-60s tomorrow, like 4 or 5 under tomorrow, that's definitely what I need, but maybe more. Like Brooks can go out and shoot 5 under, and that's the end of the rest of us. So, yeah, 4 or 5 under tomorrow.

If Lowry still has a chance, Rory McIlroy's wait for his fifth major will go into the fifth year.

Tied for 25th at halfway after picking up three shots in the last 11 holes of his weather-delayed second round to post a three under 67, the Holywood star failed to spark in the afternoon, carding a disappointing one-over 71 to finish ten shots behind Koepka.

"I'll speak to you tomorrow," McIlroy said as the trooped towards the locker room, stopping to chat for several minutes with his coach, caddie and tour manager.

His sombre mood contrasted with Woods' enthusiasm as he raced to the turn in 31 to get to within three of the lead but ran out of steam as he closed with nine pars, three-putting the 17th for par when he had a 20 footer for eagle.

"I just wish I could have got myself a couple of shots closer to the lead," Woods said. "I've got to shoot another low round tomorrow, and hopefully that will be enough."

If Woods is barely optimistic, Mcllroy knows he's simply making up the numbers after another major-less season.

He was out of sorts with every part of his game and his three birdies in the morning round came when he sandwiched a chip-in birdie at the 14th between tap-ins at the par-five eighth and 17th holes.

"I didn’t have my best stuff out there; I think I only hit one green in the last six holes," McIlroy said before the turnaround. "It felt like the only way I was going to break it was by chipping in or a tap-in putt.

"I need to tighten up my iron play this afternoon to give myself a chance."

Sadly for McIlroy, his warm-up for the third round did not go well as he shanked a wedge on the range.

And his hopes of a fast start evaporated when he bogeyed the first, carving his tee shot with an iron into the rough before flying the green and taking two chips to make the green.

He birdied the second with a towering approach to five feet, but after getting up and down from sand to save par at third, he tugged a long iron 70 feet left of the flag on the fourth and had to pitch and putt to save par after a pushed tee shot at the fifth.

ST. LOUIS, MO - August 11: Tiger Woods on the first hole during the third round of the 100th PGA Championship held at Bellerive Golf Club on August 11, 2018 in St. Louis, Missouri. (Photo by Montana Pritchard/PGA of America)

He was scrambling again at the 198-yard sixth after coming up short and left with his approach.

But after saving par there, he dropped his third shot of the afternoon with a three-putt bogey at the seventh, where he had just 122 yards to the pin from the middle of the fairway but came up 40 feet short.

Out in two-over 37, it spoke volumes about McIlroy's day that Ross Fisher won the front nine in their threesome with JJ Spaun by turning in one-over 36.

Eight shot behind leaders Woodland and Koepka heading down the back nine, he needed to make something happen and briefly sparked to life.

He poured in a ten footer at the 10th and a 16 footer at the 11th to get to within seven of the leaders on three-under.

But his mini charge ended as quickly as it started when he tugged his tee shot into the trees at the 12th, took three more shots to find the green and missed a four-footer for bogey.

That double bogey there put paid to his faint hopes of a title challenge, and while he birdied the 14th to get back to one over for the round and salvaged a par after driving into the creek at the 17th, it was a bitterly disappointing day for the Co Down man.

Woods lit up the first half of the afternoon, picking up birdies at the first and second before following a three-putt bogey at the fifth with three birdies in a row from the sixth to get within two of the lead before the birdies dried up.

All the fireworks came from US Open champion Koepka, who showed his intent early, blasting a huge drive down the middle before zipping a wedge back to a foot at the first.

Birdies followed at the second, fifth, eighth and ninth as move three clear at the turn on 13-under par.

Seemingly cruising, Kopeka had to make an eight-footer for par at the 13th but then bogeyed the 14th and 15th to see his lead reduced to two with three holes to go.

Scott finished birdie-birdie-par to get within one with a 65, but Koepka birdied the 17th to take a two-shot lead into the final day.