Rory McIlroy's brilliant US season did not get the fairytale finish he'd hope for as he produced a tired final round performance in the Tour Championship in Atlanta that cost him a chance to win the FedEx Cup and a $10m bonus.

Regretting his decision to play all four playoff events rather than take a break last week, five dropped shot in a mid-round six-hole stretch fatally holed McIlroy's title bid below the waterline.

While he birdied three of his last four holes to camouflage the result somewhat and salvage some pride with a one over 71 that left him tied for second with Jim Furyk, it will gall McIlroy that he wasn't a factor coming down the stretch.

A brilliant Billy Horschel won in back to back weeks to claim the FedEx Cup and an $11.44m payday — perfect timing for him and his expectant wife Brittany but bad timing for US Ryder Cup skipper Tom Watson in terms on the captain's picks that went to Webb Simpson, Hunter Mahan and Keegan Bradley.

Horschel, or Chris Kirk, who was second in the FedEx Cup final standings ahead of McIlroy and fourth in Atlanta, will be watching the action at Gleneagles on TV, if at all.

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"It's been a long four weeks.  You know, if I had to do it all over again, I probably would have taken a week off somewhere in this stretch of tournaments," McIlroy said after finishng three shots behind Horschel, who shot a 68 to win on 11 under. He has now earned $13.48m in three weeks.

"But you know, it's tough because‑‑ you know, these sponsors that are sponsoring the tournaments and the FedExCup PLAYOFFS, they're putting up an awful lot of money to get the best players in the world there to their event. 

"Not that you feel obliged, but you want to support the events and you want to show up and give the sponsors something to be happy about and proud of.  And you know, that's really‑‑ you know, I've got a great relationship with BMW, and that was probably the only reason why I did play in Denver. 

"But at the same time, you know, if I had to do it again, I probably would have taken a week off just to feel a little fresher coming in here."

Still, the double bogey on the sixth and the bogeys at the ninth, 10th and 11th will leave a sour taste for a few days even if he did finish third in the FedEx Cup standings in the end.

“I just got really frustrated and just couldn’t muster the energy to try and get something going again,”  McIlroy said. “And even when I was hitting good shots, I was hitting bad putts.

“I’m happy I made those three birdies coming in. It jumped me up the leaderboard a little bit and at least finished the day respectably.”

The PGA Tour numbers crunchers will tell you that "McIlroy is now five of eight when holding the lead/co-lead through 54 holes on the PGA TOUR." But this one will sting a little bit as he was hoping to put the cherry on top of a sweet season that has brought him another two majors and his first WGC.

He admitted on Saturday night that he was "weary" coming into the week but while not playing with his A game, he somehow managed to battle his way into a share of the lead with Horschel with a round to go. 

After 10 holes, he confessed afterwards, his chance of winning the tournament or the FedEx Cup had gone.

And even though he rallied at the end, it was Furyk who occupied Horschel's thoughts down the stretch.

Horschel was two clear after a front nine of 33 and after Furyk drew level with a birdie on the 15th, Horschel edged ahead once more with his own birdie on the par five minutes later.

He then holed an unlikely 30 footer for par at the 16th to remain in front and when Furyk came up well short with his approach to the 17th and bogeyed (he also three putted the last), it was all but over.

Having won the BMW Championship in Denver last week and finished runner-up in the Deutsche Bank Championship the week before, 27-year old Horschel turned a modest season into a sensational one in the space of three weeks.

He'd had just two Top-10s all season as McIlroy swept all before him and even missed the cut in The Barclays, the first of the four consecutive FedEx Cup playoff events.

A putting tip from his coach turned things around and he was an impressive and deserving winner.

Still, McIlroy was bitterly disappointed to throw the title away, starting with a double bogey five at the short sixth, followed by three bogeys in a row from the ninth to the 11th.

It looked like it might be his day when he hit a wild drie at the fourth but still made birdie from 18 feet. 

But Horschel matched him from 11 feet to remain on terms and then edged ahead on the next, where he holed from 18 feet.

Perhaps the decisive moment arrived on the treacherous par-three sixth, where Horschel hit a superb three iron into the heart of the green and then saw McIlroy pull his five-iron before two putting for a double bogey after a pitching to 12 feet from the drop-zone.

"It was just a tired swing," McIlroy told Sky Sports. "My lower body got really lazy and my upper body took over and I just missed it way left. It was just a bad swing." 

He needed a birdie at the par-five ninth but unlike the US PGA, where he made eagle to surge back into contentio, a wild slice cost him another crucial shot as it sailed miles right up against a fence.

"I'm just going to blast it, see where it goes," he told his caddie as he sized up an impossible recovery over trees.

He did well in the end to single putt for a bogey six and it was a bogey that saw him lose his cool, his composure and the tournament.

"It was the opposite of the sixth," he told Sky afterwards."I just got the club stuck way behind me and it could never catch up. I was just making bad golf swing at the wrong times out there and getting punished for them. A hole you are looking to birdie and you make six, it really killed any momentum that I had."

Now four behind Horshel, McIlroy thought he might get a shot back when the American bogeyed the 10th. But the 25-year old Ulsterman three-putted from 20 feet, missing from less than three feet for bogey. Displeased, he hurled his putter at his bag.

"When Billy was making bogey on ten I thought I had a decent chance and I three-putt that again for the second day in a row and I felt like any chance I had to win the golf tournament went. That's really where I lost it, I feel.

"Then to compound that with another bogey on 11, I just had to regroup somehow and make some birdies coming in and finish as well as I could and I was able to do that."

Asked about his season overall, McIlroy added: "To win two Majors, my first WGC event and give it a real good run in these FedEx events, I'm really proud of myself the way I hung in there and dug deep the last few weeks.

"When I look back on the year as a whole it's been my best year to date so I can't complain.

“I’m looking forward to a few days off and not seeing my golf clubs for a little while. I’ve been in the gym every day this week and that’s fine, but mentally I’m a little fatigued. Billy deserved it, he played the best golf all week.”

McIlroy should be physically perfect for the Ryder Cup but how sharp he will be remains to be seen after the most successful summer of the is career, FedEx Cup disappointment notwithstanding.

"There's a few guys I'm glad I'm not going to see at Gleneagles:  him (Horschel), Ryan Palmer, Chris Kirk.  There's a few guys that are playing well that aren't on this U.S. team that obviously had a great chance to make it."

As for FedEx Cup fatigue being a factor in Scotland, he said: "I think a week off will do a lot of us a world of good.  It really will. 

"As I said, I don't want to see my golf clubs for a few days and I'll pick them back up again probably at the weekend before. 

"Having these five days off is going to be big, and I think a lot of guys will do the same thing, and I don't think like tiredness or fatigue will play a part in the Ryder Cup."