There's still one round to go but after watching Rory McIlroy hole a 60 footer and then four-putt from less than five feet, tiredness is clearly taking its toll on the Holywood man as he battles to cap a sensational summer by winning the FedEx Cup.

A two over 72 in the BMW Championship at Cherry Hills relegated McIlroy from a share of third overnight to tied 10th as he slipped nine shots behind leader Billy Horschel at Cherry Hills Country Club in Denver.

"I started off well. I was 2-under par through seven," McIlroy said. "I was right in the tournament there. Bogeyed eight, which wasn't a disaster. It was a bit of a silly bogey, three-putting there.

"Then, the 12th hole just really derailed me. I just -- the first two putts I didn't really -- I didn't lose any concentration. I took my time over them. And I just, I completely misread the first one. Then just hit a bad putt the second, and then the third one I was just going for a tap in and just lost concentration.

"So, obviously, to go from being right there in the tournament, dropping three shots in one hole wasn't what I wanted.

"But, I just need to go out tomorrow and try and post a low one and finish as high up as possible and give myself the best possible chance going into Atlanta next week."

Horschel put last week's poor finish in the Deutsche Bank championship behind him to card a bogey free 63, holing a 29 footer at the last for birdie and a three-stoke lead over Ryan Palmer (67) on 13 under par with Masters champion Bubba Watson and US OPen winner Martin Kaymer a further shot back.

The FedEx Cup escaped McIlroy a couple of years ago and after winning The Open, the WGC-Bridgestone Invitational and the US PGA (not to mention the BMW PGA at Wentworth), McIlroy wants to win the FedEx Cup so badly that dismissed thoughts of skipping the third leg of the playoffs.

That now looks like a dubious decision considering Phil Mickelson decided to withdraw overnight when it was clear he had not chance of progressing to the Tour Championship.

The left-hander was never a fan of a four-in-a-row FedEx Cup playoff run this year (it usually features a one-week break) with the break now factored in the week before the Ryder Cup.

Mickelson wants to be fit for Gleneagles and there's a chance that his Ryder Cup partner Keegan Bradley will also be fresher than McIlroy after he took himself out of the tournament by withdrawing in a fit of angst over a ruling he got on the 18th on Friday evening.

He made six despite getting a drop for an embedded ball but was not happy that he'd taken the correct action after a fan told him afterwards that his ball had bounced rather than plugging in its own pitchmark.

Either way, Mickelson and Bradley are out of the series and McIlroy is out of contention in Colorado and facing another tough week in Atlanta next week with the $10m FedEx Cup bonus on the line.

He's clearly the game's dominant player and whether this is a new era or not, it's the first time since 1992 that Tiger Woods and Mickelson haven't won a PGA TOUR tournament in a season. 

As for McIlroy's round, he holed five footers for par at the first two holes, bogeyed the driveable third but then roared into a share of the lead with birdies at the fourth (he drained a 60 footer), par-three sixth (10 feet) and the short, par-four seventh (three feet) before he bogeyed the eighth by missing the green and a six foot putt for par.

His playing partner, overnight leader Sergio Garcia, struggled on the greens and also shot 72 to fall back to sixth, seven off the pace.

But while Garcia missed some short putts, nothing beat McIlroy's four-putt at the par-three 12th, where he missed the green by a yard or two left and took five more to get down from 30 feet.

His chip finished just four feet from the hole but he misread his par putt, knocking it nearly five feet past and then took three more putts from there with another misread followed by a power lip out.

It was his first triple bogey on the PGA Tour this year and while he birdied the next from 16 feet, he bogeyed the 247-yard 15th when he went long and left with his tee shot and could't chip closer than 35 feet.

McIlroy made it clear in the build up to the event that he was gunning for the FedEx Cup despite fatigue — it's his seventh event in nine weeks and his third in a row with another to come in Atlanta next week.

"I think it's been such a great year on the golf course that if I wasn't to go on and win the FedEx Cup then it would definitely be disappointing," McIlroy said. 

"I really want to cap off this summer as best I can.

"I have two more weeks to push through, even though I am feeling a little tired, I'm trying to conserve as much energy as possible."