Rory McIlroy launched a staunch attack on Brooks Koepka’s macho mind games as he extended his major drought to six years in the PGA Championship in San Francisco.

The Co Down man (31) made an eagle two, two birdies and two bogeys in closing a 68 at TPC Harding Park but finished tied 31st on two-under, 11 strokes behind the new champion, Collin Morikawa (23), who shot a superb 64 to win by two shots from Dustin Johnson and Paul Casey on 13-under.

It was another disappointing week for McIlroy, who has not been a serious factor in 16 of his last 20 major championship appearances and appears to have no idea why.

“No, I don't know. Maybe I'm just not as good as I used to be!” he said sarcastically.

“I don't know. I really don't know. I feel like the golf that I've played in the majors has been sort of similar to the golf I've played outside of them, and I've won some big events and played well and had a good season last season. 

“I can't really put my finger on it. I go out there and try my best every single day. Some days I play better than others, and just got to keep going and keep persisting and see if you can do better the next time.”

That his golf in the majors has been of similar quality to his golf on the PGA Tour is debatable.

And having said before the start that he needed a fast start to get into the mix, he faded away after opening with a respectable, level par 70.

The three-hole stretch from the 12th to the 14th, which he played in 10-over par, had much to do with his downfall.

“It was a bad stretch of holes for me this week,” said McIlroy, who was never a factor after making a triple bogey seven on the 12th on Friday. “But I have to be encouraged by the rest of the golf that I've played.”

Winning majors is about sticking around and avoiding big mistakes as much as it’s about making birdies — 16 for McIlroy last week.

Whatever he really thinks about what’s holding him back, he certainly has no intention of taking a leaf out of Koepka’s book of tough talking machismo and admitted he was taken aback by the American’s candid comments about Johnson on the eve of the final round.

“I like my chances,” Koepka said, clearly referencing Johnson’s multiple major disappointments. “When I've been in this position before, I've capitalisedI don't know, he's only won one.”

As it turned out, Koepka’s bravado was a bluff. After tugging his opening drive left, he was never the same and turned in four-over, eventually limping to a 74 that left him ten shots behind the winner in tied 29th.

Insisting he’d never stoop to that tactic, McIlroy said: “It's different, right. It’s a very different mentality to bring to golf that I don't think a lot of golfers have. 

“I was watching the golf last night and heard the interview and was just sort of taken aback a little bit by sort of what he said and whether he was trying to play mind games or not.

“If he's trying to play mind games, he's trying to do it to the wrong person. I don't think DJ really gives much of a… concern about that. 

“But just different. I certainly try to respect everyone out here. Everyone is a great player. If you've won a major championship, you're a hell of a player. Doesn't mean you've only won one; you've won one, and you've had to do a lot of good things to do that. 

“It’s sort of hard to knock a guy that's got 21 wins on the PGA TOUR, which is three times what Brooks has.”

McIlroy, who has 18 PGA Tour wins to Koepka’s seven, wasn’t the only big name to leave San Francisco disappointed.

Open champion Shane Lowry mixed three birdies with five bogeys and a double-bogey six at the 16th, taking 33 putts in a 74 to finish tied 66th on three-over.

But despite his struggles from the tee, he still feels positive about his game as he attempts to make the FedEx Cup Playoffs with a good performance in the Wyndham Championship this week.

“I played okay this week,” he said. “Played okay in parts. I struggled off the tee which is going to put you in trouble around here but I was grinding my nuts off the last few holes and got absolutely screwed with a lie on 16 and 18 and lipped out for birdie on 17.

“If I could have birdied two of the last three, which is doable here, it would have been a half-decent day but I played them badly and got a bit unfortunate.”

Lowry was four-under for the tournament through seven holes of his second round but the stretch from the eighth to the 14th cost him dear, leading to a spectacular club snap on the 13th on Friday.

“I felt I wasn’t far away this week, especially when you look at the leaders only nine-under at the minute,” he said, explaining that he was playing with a fade this week when the required shot shape was a draw. 

“I had four three-putts over the weekend and if I can just clean that up I can hopefully make it to the Playoffs, hopefully have a run and go to [the US Open] at Winged Foot, which is a test I am looking forward to and test I will like.”

Tiger Woods went into the final round tied for 59th but while his putting was much improved as he closed with a three-under 67 to crack the top 40 on one-under, his haul of 14 birdies was not enough to make up for his tally of 13 bogeys.

“That's golf,” Woods said. “We lose way more tournaments than we win. This is one of those weeks where I didn't quite make the putts when I needed to, and I missed a couple shots on the wrong sides. 

“But overall, I think I had one three-putt for the week and I had no doubles, and that's always something that you want to do going in throughout 72 holes of a major championship. 

“Unfortunately I didn't make enough birdies, and I'm not there with a chance come this afternoon.” 

It was also a week to forget for Phil Mickelson, who signed off with a double-bogey at the 18th for a 73 that left him tied 71st on four-over.

“I really thought after finishing second last week, I was really gained a lot of momentum,” said the left-hander, whose biggest success was in the CBS commentary booth on Saturday. “I thought I was going to come here and have a great week and I barely made the cut, and just never got it going and was just a fraction off across the board.”

Morikawa was the victor, emerging from a forest of names at the top of the leaderboard with a six-under 64.

He separated himself from the pack by chipping in for birdie at the 14th, then hit a glorious tee shot to seven feet at the 293-yard 16th, bleeding a driver left to right to set up the simplest of eagle putts.

At age 23 years, 6 months old, he is the fourth player to win a PGA Championship prior to his 24th birthday, joining McIlroy (23 years, 3 months in 2012), Jack Nicklaus (23 year, 6 months in 1963, slightly younger than Morikawa) and Tiger Woods (23 years, 7 months in 1999).

This was Morikawa’s second major start.  He’s the seventh player in the Masters era (since 1934) to win one of his first two major starts, joining Ben Curtis (2003 Open Championship, 1st major start), Keegan Bradley (2011 PGA Championship, 1st start), Orville Moody (1969 US Open, 2nd start), Fred Daly (1947 Open Championship, 2nd start) and Bob Hamilton (1944 PGA Championship, 2nd start).

“It's amazing,” said Morikawa, who played in the US team that finished second to Denmark in the Eisenhower Trophy at Carton House just two years ago. “It's been a life goal, obviously as a little kid, kind of watching everyone grow up, all these professionals, and this is always what I've wanted to do.

“I felt very comfortable from the start. As an amateur, junior golfer, turning professional last year, but to finally close it off and come out here in San Francisco, pretty much my second home where I spent the last four years, is pretty special.”